The 20-year-old signed a three-year contract with the Scottish Premiership club and becomes their sixth addition of the summer transfer window.
Sule began his career at Celtic before moving to West Brom in January 2023, where he signed his first professional deal later that year.
He made his senior debut for the Baggies in an EFL Cup clash against Fleetwood Town in August 2024.
The forward also spent time on loan at Motherwell last season, making three appearances in the Premier Sports Cup before injury ended his spell early.
"I'm delighted. This was always on my mind, to come to a team and be number nine at a club. That's amazing to me, so it was a no-brainer,” Sule told stmirren.com
"When I spoke to the gaffer, everything he was saying to me, it aligns with my values and what I want to be. I feel like this is the perfect place for my career. I'm excited.
"I see myself bringing athleticism, pace, goals and energy. Also, I'm a winner, so I want to win games. I came here to win and to show everyone what I can do and hopefully the team as well.
“Even though I had just a small taste of it before, I feel like it's a league where I can really do well and show my abilities."