"Proud to part of the St Louis Blues organization. Excited to get started," Roy said via his Twitter feed.
Roy, 30, split the 2012-13 season between the Dallas Stars and Vancouver Canucks. He had four goals and 22 points with the Stars, who acquired him from the Buffalo Sabres in July 2012. Dallas traded him to Vancouver on April 2, and he had three goals and six points in 12 regular-season games, and one assist in Vancouver's four-game loss to the San Jose Sharks in the first round of the Stanley Cup Playoffs.
"Coming to a young, skillful team like this, with a ton of grit, was right in my wheelhouse," he told the St. Louis Post-Dispatch. "I want to win a Stanley Cup, and I thought this team was on the verge of becoming a great contending team."
The Sabres took Roy with their second-round pick (No. 32) in the 2001 NHL Draft. He made his NHL debut with them in 2003-04 and played for Buffalo through the 2011-12 season. From 2006-07 through 2010-11, Roy had at least 20 goals and 63 points; his career bests are 32 goals and 81 points in 2007-08.
Roy, who is 5-foot-9 and 185 pounds, has been injury-prone in recent years. He was off to a point-per-game start before missing the final 46 games of the 2010-11 season and six playoff games after having surgery to repair a tendon in his left quadriceps. He also had shoulder surgery in July 2012.
The Blues are hoping his scoring touch returns after a disappointing 2012-13 season.
"[Roy] is a playmaking centerman and he's just 30 years old," general manager Doug Armstrong told the Post-Dispatch's Jeremy Rutherford. "I think it's a great opportunity for Derek to re-establish himself."
Roy does give the Blues the top-six center presence they were chasing during free agency. He has 168 goals and 455 points in 591 career games in the NHL, and seven goals and 26 points in 45 Stanley Cup Playoff games.