Clements, the star cornerback from the Buffalo Bills, agreed to an eight-year, $80 million contract. The six-year veteran was considered one of the top prizes in free agency, and the 49ers rewarded him accordingly with what's believed to be one of the richest contracts for a defensive back in NFL history.

Terms of the deal were confirmed by a person familiar with the negotiations. The person spoke to The Associated Press on the condition of anonymity because contract terms aren't usually released by NFL teams.

The 49ers have more than $30 million in cap room to spend on free agents, and a good chunk of it immediately went to Clements, a two-time Pro Bowl selection who spent his first six NFL seasons with Buffalo. He became a free agent after completing a one-year contract, saying he hoped to better Champ Bailey's seven-year, $63 million deal with Denver in 2004.

Lewis, a safety from Philadelphia, will become an immediate starter in the 49ers' inconsistent defensive secondary alongside Clements, coach Mike Nolan said. Lewis started in the Pro Bowl following the Eagles' trip to the Super Bowl in the 2004 season, but was expected to leave town after losing his starting job in Philadelphia last season.

"Free agency is about adding good players with character to our team," Nolan said. "I am confident that Nate and Michael will help make us better. Players have a choice in free agency. It is a compliment to our players and fans that Nate and Michael wanted to be with the 49ers."

Clements and Lewis both visited the 49ers' training complex today, and wasted no time sewing up deals to bolster a secondary that struggled with injuries and mistakes last season despite cornerback Walt Harris' Pro Bowl play.

Both of the 49ers' opening-day safeties lost their jobs during the year, and cornerback Shawntae Spencer struggled at times. San Francisco's defense yielded a league-worst 412 points and lost games by 41, 31 and 29 points during a midseason collapse, prompting Nolan to fire defensive coordinator Billy Davis.

Spencer, who got a lucrative contract extension last season, might be reduced to a nickel back with Clements and Harris in starting spots. Lewis has been a strong safety with the Eagles, but could be a candidate to replace either of the 49ers' starters: strong safety Mark Roman and free safety Keith Lewis.

The 49ers already have made a significant upgrade to a club that went 7-9 last season in Nolan's second year — and they probably aren't done shopping, either. They have long held strong interest in Pro Bowl linebacker Adalius Thomas, who played for Nolan with the Baltimore Ravens.

Clements is coming off a strong season in which he had three interceptions, including one returned for a touchdown, and recovered two fumbles. In six seasons, he has 23 interceptions, including five returned for touchdowns — and has also returned two punts for scores.

Clements wouldn't have been in a position to sign such a lucrative deal if the Bills hadn't prevented him from becoming a free agent last offseason. Buffalo labeled Clements as the team's franchise player, a move that retained Clements' rights for a steep one-year deal worth $7.2 million.

Clements accepted the one-year contract after the Bills promised him they would not tag him again this offseason, allowing him to pursue free agency.