Ottawa selected the 21-year-old in the fifth round of the 2019 NHL Draft.
"Since being drafted just last June, Mark has worked hard to earn this contract," general manager Pierre Dorion said. "We were pleased to see how he progressed in his overage season in Calgary, where he served as Hitmen captain for two consecutive seasons, and where he was on track to post his most productive major junior season. He's a solid two-way center who shows strong attention to detail in his play."
The 6-foot-3 pivot led the Western Hockey League's Calgary Hitmen in both goals (38) and points (68) through 58 games in 2019-20, his fifth season with the club. Kastelic finished his major junior career ranked third in franchise history with 126 goals.