The 22-year-old academy product has made four senior appearances for the Saints this season.
Isgrove, who spent the end of the 2013-14 campaign on loan at Peterborough United, is now looking forward to establishing himself in the first team at St Mary's Stadium.
"It's a bit of security," he is quoted as saying by Sky Sports News. "I'm 22 now so obviously this takes me to 24. It won't stop me working hard and pushing in training. That's a good age to be playing at a high level.
"It shows that they believe in my abilities. It's probably the biggest two years of my career so far. If I can get a run of games, I can really push on and really establish myself as a professional player."
Southampton are back in action on Saturday when they host Burnley in the Premier League.