Financial terms were not disclosed for the oft-injured Sehorn, who reportedly chose to sign with the Rams over Jacksonville and Carolina.
Sehorn spent his first nine seasons with the New York Giants before being released in March after failing to accept a $4.3 million pay cut.
The Rams were desperate for help in the secondary after being hit hard in free agency, where they lost cornerbacks Dre' Bly and Dexter McCleon.
Despite the losses at cornerback, the Rams are expected to move Sehorn from his traditional cornerback spot to free safety. St. Louis used Kim Herring at free safety last season.
"I think Jason will be a tremendous addition to our defense," said St. Louis Coach Mike Martz. "I look forward to getting him here as soon as possible, getting him with (defensive coordinator Lovie) Smith, and the rest of the defense, and learning what we do."
In recent seasons when the Rams and Giants have played, Martz never showed much admiration for Sehorn's coverage ability.
In his final season with the Giants, Sehorn was relegated to a nickel back after losing his starting cornerback job to William Peterson. The demotion came just two years after Sehorn helped the Giants reach Super Bowl XXXV.
The second-round pick in 1994 out of Southern California was a reserve and special teams player his first two seasons in New York. In 1996, he became the starting right cornerback, and led the team with five interceptions, including a 23-yard touchdown.
The following season, he started all 16 games for the only time in his career. He had a career-high six interceptions along with 86 total tackles and 20 passes defensed as the Giants captured the NFC East title.
He missed the entire 1998 season after tearing the ACL and MCL in his right knee while returning the opening kickoff in a preseason game against the New York Jets on Aug. 20.
Sehorn returned to play and start in 10 games in 1999. He missed the first two games because of a pulled hamstring, and the final four games because of a fractured left leg suffered against the Jets on Dec. 5.
In 2000, he started 14 games, missing two because of a fractured rib suffered when intercepting a pass in an Oct. 15 game against Dallas. He had 77 tackles, two interceptions and a team-high 17 passes defensed.
In the NFC championship game, Sehorn held Minnesota Pro Bowl wide receiver Randy Moss to two catches for 18 yards, and had an interception in the end zone. In the divisional playoff, he had one of the most memorable interceptions in Giants' history when he dove for a pass by Donovan McNabb intended for Torrance Small. Sehorn tipped the ball into the air, jumped off the ground, caught the ball, and raced 32 yards into the end zone.
Sehorn played poorly in Super Bowl XXXV, but was given a six-year, $36 million contract. Not long after, in a high-publicized story, he married actress Angie Harmon.
In 2001, Sehorn started 13 games and led the team with 16 passes defensed and tied for second with three interceptions, including one he returned for a touchdown.
In his career, Sehorn has totaled 422 tackles, 358 solo, 19 interceptions, four for touchdowns, 83 passes defensed, 10 fumble recoveries, and three forced fumbles. The 10-year veteran has also played in four postseason contests, starting all four, adding 16 solo tackles, three interceptions (one for a touchdown), and seven passes defensed.
The 6-2, 213-pounder was a second-round draft pick by the Giants out of Southern California in 1994. While in New York, he was active in the Northern New Jersey community, establishing his own foundation, Sehorn's Corner, that assists single-parent families move into new, fully-furnished homes.