He was known to be a billionaire, but at the time there was no indication as to how much money the Russian was prepared to pump into the London club.
The big signings arrived soon after, though, and so did the major trophies. He may have sacked more than his fair share of managers, but in terms of transfers, Abramovich has walked the walk.
Below, Sports Mole has picked out 10 of the best pieces of business that the 46-year-old has sanctioned.
He may have only been at Chelsea a year, but the young Belgian has made a real impression on the Stamford Bridge faithful. What's more, at the age of just 22, Hazard is only going to improve. The attacker played a key role during last season's Europa League success and managed to score 13 goals in all competitions. The youngster cost a hefty £30m, but it already looks as though he will be a sound investment.
During his first two seasons at the club, the Serbian international made just 26 appearances in all competitions. It resulted in reports that he would leave England, with AC Milan believed to be keen on his services. However, from 2009 onwards, Ivanovic has been a regular in the Chelsea defence. His no-nonsense approach has seen him become a favourite of the fans and it has also been aided by his knack for scoring goals. He has scored none as important as the stoppage-time winner in the Europa League final.
The Dutch international may have lasted just three seasons at Chelsea, but for two of those at least, there were few better wingers in world football. Despite his injuries, the reported £12m signing was a key component of the side that won back-to-back Premier League titles under Jose Mourinho. Although his primary role was to set up his teammates, Robben would often weigh in with important goals.
He may have commanded a fee in the region of £20m, but Carvalho proved himself to be worth every penny during his six years with the club. He arrived as one of the best defenders from that summer's European Championships and went on to form a strong partnership with John Terry. During his first campaign, the Blues conceded just 15 goals in 38 Premier League matches. He will also be remembered for the goal that he scored during the 3-0 win over Manchester United that clinched the title in 2006.
For his sheer effort and desire, Essien is a hero to some at Chelsea. The Ghanaian is also an able footballer and has shown that on countless occasions down the years. In recent seasons knee injuries have prevented him from being at his dynamic best and he spent last year with Mourinho in Madrid. They are both back now at Stamford Bridge, though, and the 30-year-old will be hoping to add to his two Premier League titles and four FA Cup wins.
The Frenchman was so good that the position that he played in - holding midfield - was named after him. In fact, a large number of Real Madrid supporters were bemused when their club sold him to Chelsea for £16m. His presence at the base of the midfield allowed the likes of Frank Lampard, Damien Duff and Robben to go and parade their attacking flair on proceedings.
If someone has paid £23.5m for your signature, the pressure is on to deliver immediately. That was no problem for Mata, who scored on his debut against Norwich City and hasn't looked back since. In two seasons, the Spaniard has scored 32 goals and made a remarkable 59 assists. All of which makes talk of Mourinho being prepared to sell him rather unbelievable and bizarre.
The way that the England defender's move to Chelsea come about may have left a bitter taste in the mouth, but there can be no doubting how well he has done for the club. He only cost £5m, with William Gallas moving in the opposite direction to Arsenal. Since then he has won virtually ever prize on offer and still at 32 is regarded to be one of the best left-backs in the world.
Not too much was known about Cech when he arrived nine years ago, but during his debut campaign he set a Premier League record of 1,025 minutes without conceding a goal. His time at the Bridge has largely been on an upward spiral, although he did spend a while on the sidelines with a serious head injury following a clash with Stephen Hunt at Reading. His finest moment came during the 2012 Champions League final when he saved a penalty from Robben in extra time, before stopping two more during the shootout.
He may have gone to ground far too easily for a man of his size, but there probably isn't a Chelsea fan who doesn't hold Drogba in the highest regard. Even when big money was spent on Andriy Shevchenko and Fernando Torres, the big Ivorian battled back to reclaim his place. His winning penalty in the Champions League final will forever remain one of the best moments in the club's history.