Spikes had been talking to the 49ers and the Detroit Lions for the past couple of weeks but interest intensified with San Francisco over the weekend. When the team released starting inside linebacker Brandon Moore on Sunday, Spikes became interested enough in the 49ers to say yes to coach Mike Nolan and no to Lions general manager Matt Millen.
Terms of the deal weren't available. Months ago, Spikes turned down a one-year, $1.5 million offer from the Lions, according to a source.
Spikes is expected to start alongside Patrick Willis, the NFL's reigning defensive rookie of the year, at strong inside linebacker in the team's 3-4 defense.
"It's going to be one of the strongest linebacking corps in the middle with him and Patrick," said defensive end Justin Smith, who teamed with Spikes for two seasons in Cincinnati.
"Great guy. As far as football goes, he's just a leader. He's got that quality about him. He knows his stuff. He pays attention to his film study. He's prepared. He's the type of guy you need on a defense. That's a plus."
Spikes spent 10 seasons with the Cincinnati Bengals, Buffalo Bills and Philadelphia Eagles. His season with the Eagles last year ended with a torn rotator cuff in the 14th game. He finished with 85 tackles, including 63 solo stops, and one sack.
The interesting thing is that Spikes is going to a 3-4 defense where he is slated to play inside linebacker. Spikes gained a reputation as one of the league's top weakside linebackers in a 4-3 scheme. The 13th pick in the 1998 draft, Spikes played for the Bengals, Bills and Eagles during his career. He went to the Pro Bowl in 2003 and 2004 after signing with the Bills. Last year, he played for the Eagles. The Lions were offering a strong side, outside linebacker position in their defense.
In the end, Spikes preferred the chance to join the 49ers.
Moore started 10 games in 2005 and 11 in 2006, when he led the team in tackles with 114 and sacks with 6½. He lost his starting job last year.
John Clayton is a senior NFL writer for ESPN.com. Information from The Associated Press contributed to this report.