Stephens has impressed since the New Year alongside Maya Yoshida in the Saints defence.
The 23-year-old had spent time out on loan at Swindon, Middlesbrough and Coventry and, last summer, his St Mary's dream looked to be hanging by a thread, writes Peter Howard.
"In pre-season I did feel like this was my last chance to really show what I can do at this football club," he said.
"Obviously with the new manager coming in he did say to me that there's a lot of things I need to work on to be able to play in his team.
"I went away to do that and I feel like I've improved and I'm grateful for the chance I'm getting."