The 20-year-old hooker becomes the sixth academy player to step up to the senior squad ahead of the 2026/27 campaign, joining Jack Oliver, Macenzzie Duncan, Matthew Urwin, Johnny Ventisei and Fergus Watson in making the transition.
Stephen has enjoyed a rapid rise since linking up with the academy during the 2024/25 season, making an immediate impact on his professional debut away to Leinster in the final match of the regular campaign.
A product of Garioch RFC and Glenalmond College, the front-rower has gone from strength to strength in his first full season in Glasgow colours. He has made 15 appearances to date, featuring in all five of the club's Investec Champions Cup fixtures so far this term.
He marked his maiden try in memorable fashion, crossing the whitewash against Edinburgh at Hampden Park in December 2025, before playing a key role in the Warriors' European campaign. His score from a rolling maul against Saracens at Scotstoun secured a 28-3 victory and ensured a perfect 20-point return from the pool stages.
His form was further recognised last month with a call-up to the senior Scotland squad ahead of their final Guinness Six Nations fixtures against France and Ireland.
Speaking to glasgowwarriors.org, Stephen said: "I'm absolutely delighted.
"To be able to sign a professional deal with the club is a dream come true, and I'm really looking forward to it.
"When I first came into the academy last year, I never thought for a second I'd be in this position a year later. I thought I'd do my two years in the academy and then hopefully earn something at the end of that stint, so I'm over the moon with how it's all come about.
"I've had a bit of time to reflect on some of the big games I've had the privilege of being involved in, and it's honestly incredible. To be playing against the [British & Irish] Lions captain in Maro Itoje when we played Saracens, or a World Player of the Year in Antoine Dupont when we played Toulouse, it's been pretty surreal! Hopefully there are more experiences to come.
"Everyone here has been great with me, especially in the forwards. There's so much experience in our pack and it's been brilliant to be able to come in and learn from guys who are established internationals.
"I want to keep pushing to be involved throughout the business end of the season, and put myself in the best position possible to progress both with this club and hopefully with Scotland."
Head Coach Franco Smith added: "Since coming into our academy, Seb has applied himself well and been fully committed to becoming the best version of himself as a Glasgow Warrior.
"He brings a real energy and physicality every time he runs onto the field, and has taken every opportunity he has earned during his time here to date.
"His desire to learn from those around him and improve his own game is evident day in, day out, and we're looking forward to seeing him continue to develop over the next couple of seasons."