Turnbull, who will be 19 in October, has successfully made the step up into the Saints Under-21 side after playing over 20 times for the Development Squad during 2012/13.
He and Jack Stephens have formed a blossoming centre-back partnership in the 21s on the back of Turnbull's second year as a full-time scholar with the Club.
Measuring in at over six feet tall, Jordan is an accomplished defender, both good in the air and on the ground, where his main strengths lie in one-v-one battles.
One of his best performances for the U21s last season was in a 1-0 victory at Sunderland, where he helped preserve a clean sheet by keeping England frontman Connor Wickham quiet in a tricky match away from home.
In the second half of the campaign, Turnbull was a regular in the Elite Group, playing every minute of 11 games, including memorable victories over Arsenal and West Ham United which kept Saints in contention for a play-off place.
He also played on four occasions for the Under-18s to maintain his development.
His form over the course of the season led Turnbull to join fellow Saints man Calum Chambers in the England Under-19s squad at the end of the campaign.
Jordan played both of the opening games in the Elite Round of qualification for the UEFA European U19 Championships but, despite going unbeaten in their three matches, the Young Lions missed out to Georgia in progressing into the finals.
In signing a two-year extension to his contract with the Club, former Swindon man Turnbull follows Stephens in committing his immediate future to Southampton.
This news comes as further evidence of the Club’s ambition to give its young players the opportunity to develop thanks to the security offered by a long-term contract as it seeks to build an exciting future of home-grown talent.
Jordan tied up the deal on Tuesday afternoon at St Mary’s Stadium, and immediately set his sights on pushing on to play at the highest level with the Club.
"I'm really pleased and I think it’s a great opportunity now for me to keep driving and push on into the First Team," he said.
"As you can see with the likes of James Ward-Prowse, Luke Shaw, and Jack Stephens – who signed a new contract recently – the Club is trying to bring youth through, and that's really good for us so hopefully now I can follow their steps.
"It’s a relief that I’ve been offered a new contract, and now I can focus on football.
"My scholarship has gone very quickly, but I am pleased with how I’ve progressed, and I’ve just got push on and on now to get into the First Team."
First Team Manager Mauricio Pochettino revealed that was pleased to see Turnbull follow Jack Stephens in agreeing new terms:
"I watched a number of the Under-21s' games towards the end of the season and, after securing Jack Stephens for an extended period, I was keen to do the same with Jordan.
"He has a strong presence and worked well not only with Jack, but the rest of the team too.
"I have spoken regularly with the development coaches at the Club who also rate Jordan highly. He has progressed very well in a short space of time, and I am pleased that he now has the opportunity to challenge the senior players."