Read below to find out how Ronald Koeman's side secured their progress.
6.45pmGood evening everyone. It's a welcome distraction for some, while others prefer to focus their efforts elsewhere, but one thing is certain. The Capital One Cup returns tonight, and I will be bringing you live text coverage of Southampton's trip to the Emirates Stadium to face Arsenal.
6.47pmSouthampton have shown signs of hitting top form in recent weeks, and follow Saturday's win over fellow high flyers Swansea City with a tricky visit to North London. However, the Gunners will be confident too after returning to winning ways against Aston Villa on the weekend. We could be in for an exciting night.
6.49pmMuch of the talk prior to this tie has been about the return of Abou Diaby, who will start in the Arsenal midfield following a long battle with injuries. Can he offer a reminder of his qualities this evening? Full teams news is on the way.
6.54pmIt's a mixture of youth and experience from Arsene Wenger tonight. Diaby does start after playing just once last season, while David Ospina makes his first start in goal for the Gunners. There's also a place in the centre of defence for Isaac Hayden as only Calum Chambers keeps his spot from the weekend.
SUBSTITUTIONS: K. Davis, Long, Cork, Bertrand, Mayuka, Reed, McCarthy
6.59pmRonald Koeman makes five changes tonight, but the inclusion of a number of key players suggests that he does fancy his chances in this competition. Summer signing Sadio Mane makes his first start in attack alongside Graziano Pelle, while Florin Gardos, Steven Davis, Victor Wanyama and Matt Target are all handed spots in the team.
7.03pmA quick look at the bench confirms that a few of Arsenal's key man have been given a rest tonight. There's no place in the squad for Mesut Ozil, Aaron Ramsey, Wojciech Szczesny or Danny Welbeck. It's a great chance for the likes of Lukas Podolski and Joel Campbell to impress Wenger.
7.06pmWenger responded to the speculation surrounding Diaby's return by revealing his admiration for the midfielder, claiming: "He is a giant, mentally. To do what he has done, with the severe injuries he had, he comes back every time, is dedicated and he deserves a lot of respect."
7.08pmCan Southampton end a torrid run against the Gunners this evening? The Saints have not beaten the Gunners in the past 10 meetings, claiming four draws and losing six of those matches against Wenger's side.
7.15pmChambers will actually be the senior man at the back for the hosts tonight. We've often seen Wenger turn to his young stars over the years in this competition, but how will they get on tonight? Compared to facing Borussia Dortmund away from home, this should be a far more comfortable evening for Hector Bellerin at right-back. Francis Coquelin would perhaps prefer to be used in midfield, but he'll be hoping to remind Wenger of his capabilities on the other side of defence. The biggest test could be for Hayden, who has a big reputation, but will be in the firing line at centre back against the in-form Pelle.
7.24pmAnother statistic which will be pleasing to see for Arsenal supporters is that Wenger's side have suffered a home defeat in a domestic competition since losing to Chelsea in the fourth round of this cup last season.
7.27pmOn another night, Southampton would be big underdogs to take a win away from the Emirates, but perhaps not this evening. Wenger's decision to name an inexperienced defence could be the turning point. Koeman has decided to keep Pelle in his side, and how he combines with Mane will be crucial.
7.29pmPREDICTION: It's that time once again. I think this really is one of the toughest matches to call in the third round this week. The selections of both managers have made it even more difficult, and I think we could be in for a long night. I'm going for a 2-2 draw. Southampton to nick it in extra time.
7.32pmMauricio Pochettino received some criticism for not targeting the cups more seriously last season, but Morgan Schneiderlin has suggested that a trophy is in the squad's sights now. The French midfielder told reporters: "The players at this club want to do something great in the cup, it's very important to us. It's the shortest way to get into the Europa League, which would be great for the club and for the fans."
7.35pmKickoff is just 10 minutes away in North London. The players have gone back inside following their pre-game warm-ups, and the fans are arriving quickly now. It's unlikely to be a sell-out tonight, though.
7.39pmFor those of you who like to keep track of these things, Keith Stroud is the man in the middle with the whistle tonight.
7.42pmBoth sides are out on the pitch now. Just the handshakes to get out of the way before we see the action begin!
7.45pmIt's Tomas Rosicky who has the armband on for the Gunners tonight. The hosts will get us underway.
2 minLively start to this match as both sides look to control possession. It's Alexis Sanchez who is being asked to operate in a central role for the Gunners.
4 minGood football from Mane as he earns himself a bit of space, before winning a free kick under pressure from Sanchez. Lovely football on show early on.
5 minSouthampton are refusing to take a step backwards in the early stages. The visitors have started the match very quickly, and are passing the ball very nicely at the moment.
6 minPlenty of eyes on Diaby tonight. He's started the game pretty well, breaking up play with a couple of attacks and regularly linking with the likes of Wilshere and Rosicky.
7 minGood goalkeeping from Forster. The England international is off his line very quickly to deny Sanchez a chance from Diaby's clever chipped pass. Southampton can't afford to see the Chilean find that space too often.
9 minSAVE! Southampton create the first clear chance as Tadic slips in a great pass for Davis, whose low strike is well blocked by Ospina. Good save from Arsenal's new boy.
10 minArsenal have been given an early warning by the visitors, but both sides are looking capable of breaking the deadlock at any point soon. It's been a great start to this match.
11 minSAVE! Easy save for Forster as he responds well to keep out Rosicky's header following good work from Wilshere on the wing.
15 minYou won't see a better goal than this one tonight! The Gunners take the lead when Podolski nudges a free kick short to Sanchez, who finds the top corner with a stunning effort to give Forster no chance. What a goal from the Chilean.
16 minThat's exactly what Arsenal fans were hoping to see when Sanchez arrived at the club in the summer. The Chilean has looked lively early on, and he's been receiving far too much of the ball for Koeman's liking. Can the Saints respond?
18 minSouthampton still look comfortable on the ball, but that goals has given the Gunners plenty of confidence. The pace of this match has been frightening.
19 minPENALTY! Southampton are gifted a way back into the game. Mane cuts into the box from the left, and a dangling leg from Rosicky is punished. Silly tackle, despite there not being much contact.
21 minTadic looks very calm as he strokes the ball home from 12 yards. We're back to square one at the Emirates Stadium. The visiting supporters are delighted.
23 minBellerin saves Arsenal as the Saints threaten to go ahead moments after levelling the scores. It's good work from Pelle as he diverts Targett's ball into the direction of Mane, who is dispossessed before he can pull the trigger inside the box.
25 minIt's all Southampton at the moment. Clyne does very well on the right to earn himself some space, before delivering an inviting cross towards Pelle, who is denied a certain goal by a vital touch from Chambers. Arsenal need to relieve some of this pressure.
26 minYELLOW CARD! The first booking of the game is handed to Wanyama, who felt obliged to bring down Sanchez as the Chilean looked to sprint into space.
27 minCHANCE! Southampton come even closer to taking the lead as Tadic and Clyne combine superbly to pick out Pelle, but the Italian can only fire just wide with Ospina looking helpless.
29 minPelle has been very impressive once again tonight. Southampton have passed the ball around excellently over the past couple of seasons, but the striker seems to have added a cutting edge to their play. He's always available to get involved in the build-up, and is a physical match for anyone.
31 minArsenal are coming back into the game now. They're seeing more of the ball, and the battle in midfield has resumed. Southampton have threatened the most when targeting the hosts from wider positions, with Clyne and Targett both on top form so far.
33 minFrustration for the home supporters as a promising move breaks down on the edge of the box. Wilshere is doing his best to drive the Gunners on, but they've struggled to get the likes of Campbell and Podolski involved.
36 minVery good football from Bellerin, who shows the desire to get forward from right-back as he exchanges a couple of passes with teammates before finding space for himself. The end product is a dangerous cross, but Fonte does well to clear the ball. Campbell played a key role in that move.
38 minSouthampton are putting the pressure on once again. They've been the better side, but need to turn a good spell into a second goal.
41 minCan you remember me telling you no one would better Sanchez's effort tonight? Well, you can forget that. A Tadic corner is cleared to Clyne, who has only one thought in his mind as he fires an unstoppable effort home from around 30 yards. It's a cracking strike. Southampton deservedly lead!
42 minCHANCE! Arsenal come close to making an immediate response to falling behind, but Forster keeps the Gunners out by blocking Podolski's effort at the near post. Important save from the Saints goalkeeper.
43 minKoeman will know how important it is for his side to remain in front heading into the interval. Southampton have looked dangerous in the final third of the pitch, while Arsenal have often lacked a cutting edge to their play.
45 minArsenal finish the first half well, but another opportunity escapes them as Wilshere hits a tame effort straight at Forster following excellent work from Sanchez. Two minutes of added time to be played.
45+2The battle is going on in midfield during stoppage time. Southampton are still leading, and it's impossible to say they don't deserve to be ahead.
8.36pmWhat a first half of football at the Emirates Stadium. The game has been played at a thrilling pace since the first whistle, and we've seen two wonder strikes. Arsenal did lead through Sanchez's effort from 25 yards, but a Tadic penalty levelled the scores before Clyne handed the Saints a 2-1 advantage with a stunning strike. Can the visitors hold on for a spot in the next round?
8.38pmIf Arsenal do go out of the competition tonight, some will point to Wenger's decision to field an inexperienced defence as the turning point. In truth, they've done very little wrong, but Southampton have been very impressive going forward. Clyne and Targett, in particular, have looked threatening when attacking from deep.
8.42pmPerhaps surprisingly, the Gunners have lacked a moment of quality in the final third of the pitch. Aside from Sanchez's brilliant goal, the decision making has often been poor. When they have created chances, Forster has been a source of frustration with a couple of good saves.
8.44pmWill either manager consider making changes at the break? I'd imagine Koeman is pretty happy with what he's seen so far, but Wenger could be tempted to turn to Alex Oxlade-Chamberlain or Santi Cazorla for another creative mind in midfield.
8.46pmYou can now read a full report of the first half action at the Emirates Stadium. We're just a few moments away from the action getting back underway.
46 minNo changes yet from either manager. Southampton get the second half started in North London.
47 minBig reaction from the home supporters as Arsenal make a promising start to the second half. Sanchez, who has probably been the best player for the Gunners tonight, is looking determined to make an impact.
48 minIt's a better crowd than I was expecting tonight, with just under 60,000 supporters inside the ground tonight. Around 5,000 Saints fans have made the trip to the capital.
49 minLightning doesn't strike twice on this occasion as Sanchez hits a free kick wide of the target moments after Rosicky had fooled the referee with a dive under pressure from Schneiderlin.
51 minSAVE! Great work once again from Targett again as his cross is flicked to Davis by Tadic, with Ospina forced to make an excellent save to prevent the hosts from falling further behind.
52 minEnd-to-end stuff now as Sanchez leads another Arsenal attack. He attempts to find Campbell inside the box, but the Costa Rican runs out of space as the Southampton defence clears the danger.
53 minCHANCE! Poor from Podolski as he allows the impressive Targett to launch another Saints move forward. The ball eventually reaches Davis, whose cross is headed just over the crossbar by Gardos. Southampton are on top, but they might regret not taking their chances.
55 minKoeman will be absolutely delighted with what he is seeing from Southampton. Not only have they looked dangerous throughout the match, the visitors are pressing high up the field and restricting the influence of Wilshere, Rosicky and others.
58 minArsenal starting to pass the ball better now, but it's mostly coming in areas which will not worry Southampton. Schneiderlin, a reported target for the Gunners in the summer, is too strong in the box for Campbell, whose optimistic appeals for a penalty are dismissed.
Here's a shot of Clyne firing the Saints into the lead earlier tonight.
63 minA poor challenge from Diaby on Mane gifts the visitors a free kick on the edge of the box. However, Tadic's low strike is pushed to safety by Ospina, who has been very busy on his first start for the Gunners.
65 minArsenal just can't seem to take control of this game for an extended period. In truth, the match is being won in midfield by Southampton, despite Wenger playing the experienced trio of Wilshere, Rosicky and Diaby in that area. Diaby is definitely starting to tire on his return to first-team action.
66 minSUBSTITUTION: Wenger clearly feels that Diaby has done enough, too, as he sends on Cazorla to replace the French midfielder. Good reception from the home fans for Diaby.
68 minArsenal's first change of the match almost has an immediate impact when Cazorla delivers an excellent cross into the box, but none of his teammates take the risk as the ball drifts away from the danger zone.
69 minSAVE! More good work from Cazorla as he finds the run of Sanchez, whose effort from six yards out is well saved by Forster. A yard either side of the goalkeeper, and the scores would be level.
70 minSouthampton attack at pace following a good spell of pressure for the hosts. The ball is shifted to Mane, but he can only fire wide of the post after finding the space to unleash a shot inside the box.
71 minSUBSTITUTIONS: Oxlade-Chamberlain is off the bench to replace Campbell, before Koeman uses his first change to bring on Shane Long, with Mane jogging off after an impressive debut.
73 minThere's been a slight change to Southampton's approach, but I'm not sure if it's deliberate. The visitors are defending much deeper now as they look to trouble the Gunners on the counter-attacks. Long's introduction means they can afford to hit the channels more often.
75 minSHOT! Hayden attempts to spring a surprise by advancing forward before drilling a speculative effort wide of the target from 25 yards.
77 minArsenal dominating possession, but finding it difficult to break down a stubborn Southampton defence as Fonte makes a brilliant interception to deny Podolski as the German looked to head at goal from a tight angle.
79 minDavis is furious with the referee after colliding with Wilshere on the edge of the box. Sanchez steps up to take the free kick, but he fails to clear the wall on this occasion.
82 minArsenal are running out of time at the Emirates Stadium, and they haven't really done enough to force an equaliser in the second half. Southampton have looked comfortable for long periods since taking the lead, but there could still be a late twist.
83 minFonte will be competing with Targett, Clyne and Schneiderlin for the man of the match award if the score stays the same. The central defender makes two more crucial clearances under pressure as the Gunners desperately look for a way through.
84 minSUBSTITUTION: Targett, who has really impressed this evening, makes way for Ryan Bertrand after requiring treatment for what looks like cramp.
86 minCHANCE! Southampton are agonisingly close to wrapping up the victory as Pelle rises highest to head a Davis corner over the crossbar.
87 minSUBSTITUTION: No more time to waste for Wenger as he uses his final change to replace Bellerin with Chuba Akpom.
88 minCHANCE! Forget the scoreline, it's Southampton who look most likely to find a late goal at the moment. Pelle and Tadic link superbly well, but the latter delays shooting and is eventually tackled by Oxlade-Chamberlain.
89 minSouthampton's supporters are in great voice as time continues to run away from the Gunners. Frustratingly for Wenger, it's the visitors who are passing the ball around comfortably.
90 minThe fourth official confirms that there will be four minutes added on.
90+2 minYELLOW CARD! Wilshere takes one for the team as he chops down Schneiderlin, who was looking to launch a counter-attack for his side.
90+3 minYELLOW CARD! The referee is taking centre stage in stoppage time as he dips into his pocket again to caution Forster for time wasting.
90+4 minYELLOW CARD! All of these stoppages are not helping Arsenal. Pelle takes the sting out of another Arsenal move by bringing down Wilshere, and he picks up a booking. Southampton are just a few moments away from beating Arsenal for the first time in 10 matches.
9.40pmIt's all over at the Emirates Stadium. Arsenal miss out on a spot in the fourth round for the first time in 12 years as Southampton secure a deserved 2-1 victory in North London. The travelling supporters are delighted as Koeman walks down the tunnel with a big smile on his face. It's Clyne's stunning strike which has decided this tie.
9.41pmSouthampton march on following a thoroughly entertaining Capital One Cup tie tonight. Arsenal will now focus their efforts elsewhere, but the Saints will be quietly confident of going far in this competition. Thanks for joining me. Goodnight.