"Keith is a veteran leader who played a key role with our club's success this past season," Blues president John Davidson said Friday.
Sources told ESPN.com's Pierre LeBrun that the deal is worth $2.15 million.
Tkachuk, 37, is a five-time All-Star who has spent eight of his 17 NHL seasons with the Blues, though he was traded to Atlanta for 18 games at the end of the 2006-07 season before re-signing with St. Louis.
In 2008-09, he had 25 goals, 24 assists and 61 penalty minutes. He has 525 goals and 508 assists for his career, along with 2,163 penalty minutes.
Tkachuk has scored 20 goals or more in 15 of his 17 seasons. Last season, the Melrose, Mass., native became just the seventh U.S.-born player to reach 1,000 career points, and just the fourth player with 1,000 points and 2,000 penalty minutes for his career. He led the NHL with 52 goals in 1996-97, the first American-born player to do so.
Tkachuk ranks high on several lists for the Blues -- eighth in points (395), sixth in goals (195), seventh in game-winning goals (28) and fourth in power play goals (91).
Tkachuk was originally selected by Winnipeg in the first round in the 1990 NHL draft (19th overall). He spent five seasons with the Jets and five with Phoenix before being traded to the Blues late in the 2000-2001 season.
Tkachuk played for the U.S. during the 1992, 1998, 2002 and 2006 Winter Olympics, as well as the 1996 and 2004 World Cup of Hockey. He helped the U.S. win the World Cup championship in 1996.
Pierre LeBrun covers the NHL for ESPN.com. The Associated Press contributed to this report.