Alexander, a running back out of Alabama, joined the Seahawks as the 19th overall selection in the 2000 NFL Draft.
He helped Seattle win the last four NFC West division titles, as he became the franchise's career leader in rushing yards (9,429), attempts (2,176), total touchdowns (112), and 100-yard rushing games (37).
Alexander owns or shares 37 club single-game, single-season and career records. His 100 rushing touchdowns are tied with Marshall Faulk for seventh most in NFL history.
Alexander, a three-time Pro Bowl selection, was the first Seahawks player to earn NFL MVP honors after breaking the NFL record for most touchdowns in one season in 2005 when he scored 28. San Diego's LaDainian Tomlinson set a new record with 31 TDs in 2006.
"This was an extremely difficult decision," said Seahawks President/General Manager Tim Ruskell.