The team announced the deal Sunday. The contract runs through the 2023-24 season and carries an annual cap hit of $1.35 million.
"I want to be here. I wanted to get this done to have an opportunity just to focus on playing and just helping the team,'' Stillman said. "From what I've seen over here the last two weeks, I love the city of Chicago. The organization all the way through is top notch, and I'm really excited to be here. So I wanted to get that done, get it behind me, and just focus on the team."
The 23-year-old Stillman was acquired in a multiplayer trade with Florida on April 8. He has 16 hits and eight blocked shots in six games with Chicago.
"Having a young, physical defenseman like Riley in the fold is exciting for the future of our team," general manager Stan Bowman said in a news release. "He is growing into an excellent all-around defenseman and that continued development is something we look forward to seeing firsthand. We love his size, and his toughness adds another piece to our defense."
Stillman was selected by Florida in the 2016 NHL draft. He appeared in 34 games with the Panthers last season, finishing with five assists. He also skated in three playoff games last year.
Stillman's father, Cory, played in the NHL for 16 years, winning the Stanley Cup twice.