Seattle made the roster move Wednesday. The Seahawks released rookie linebacker John Lotulelei to clear a spot on the 53-man roster for Daniels.
The 49ers waived Daniels on Tuesday after they decided to sign John Skelton, who officially joined the team Wednesday.
San Francisco coach Jim Harbaugh said he was hoping to re-sign Daniels to the practice squad and the 49ers had plans for developing Daniels, but the Seahawks swooped in and signed him first.
Skelton was cut by the Bengals on Aug. 31 when they set their 53-man roster.