The team announced Monday afternoon that Williams has been released.
In the offseason, Williams signed a three-year, $18 million deal with $7 million guaranteed. He was slated to replace Byron Maxwell, who left as a free agent.
Seahawks coach Pete Carroll was asked why things didn't work out for Williams.
"It's a short amount of time to try and catch up with all of the real specifics of our techniques and stuff," Carroll said. "But he did a nice job. He progressed with it. But we feel more comfortable with the guys that have been with us, and so we made the move.
"He was a real pro. He worked hard in practice and all that. His attitude was excellent."
Carroll added that cornerback Jeremy Lane now being healthy factored into the team's decision.
Williams, 30, was inconsistent through the first 10 games. Coach Pete Carroll benched him during a Week 11 matchup against the San Francisco 49ers, and Williams was replaced by DeShawn Shead and was inactive the next two games.
He has started 75 games in his eight-year career. After breaking in with the Tennessee Titans and Baltimore Ravens in his rookie season, Williams spent three more years with the Ravens before signing with the Philadelphia Eagles as a free agent. The Eagles released him before his contract was up last offseason.
The move provides the team with no financial relief since Williams' salary for 2015 is guaranteed. Carroll was asked why the Seahawks did not keep Williams until the end of the season for depth purposes and indicated only that the team needed his roster spot.
No corresponding move was immediately announced.