The former Falkirk midfielder could secure another year’s contract beyond next term having also agreed an appearance-based extension clause.
The 26-year-old made the switch from the Bairns to Saints in 2016 and has gone on to make 71 appearances over the past two campaigns.
Alston told the club’s website: “I’m delighted to be staying at Saints next season and I’m really enjoying it here.
“I’ve loved my football ever since I arrived and although the injury against Aberdeen has been a blow and spoiled the final few weeks of this season, it’s still been an enjoyable campaign.
“I’ll be ready for the start of preseason and I’m really looking forward to getting back involved and helping the club kick on.
“I feel I’ve contributed well to the squad over the past two seasons but one thing I would like to improve is my goal return so that’s something I’ll be looking to add to my game next season.”
Alston has missed the McDiarmid Park side’s Ladbrokes Premiership run in after rupturing ankle ligaments at the end of March but boss Tommy Wright is looking forward to seeing him report back for action in preseason.
He said: “We’re delighted to get this one sewn up and have Blair here for another year.
“It’s been a frustrating campaign for him after being injured at the start and now the end of the season, but when he has been fit I feel like he has contributed well to the team.
“We played Blair out wide for most of the season but I’m hoping that next year we can bring him into a more central role and utilise him there.
“I think it’s a great piece of business by the club and we can look forward to having Blair back fit and ready to go for the new season.”