Spiller's deal is worth $25 million, but could escalate as high as $37.5 million if incentives are met, a source told ESPN.com's John Clayton. According to the source, the contract also includes $20.8 million in guarantees.
Spiller arrived at the team's training camp facility in suburban Rochester at around noon to sign his contract, and then headed directly to attend team meetings. He's scheduled to take part in the Bills' second practice of the day, which starts at 7:30 p.m.
Through Friday morning, he'd missed 11 practices since camp opened July 29.
Spiller joins left tackle Russell Okung, the Seattle Seahawks' pick at No. 6, as the final first-rounders to agree to terms.
Okung agreed Friday to a six-year, $48.5 million, including more than $29 million guaranteed, sources told ESPN senior NFL analyst Chris Mortensen.
Spiller, selected ninth overall out of Clemson, took part in all of the team's offseason minicamps in May and June. But he has some catching up to do, with Buffalo installing a new offense under first-year head coach Chan Gailey. Spiller is expected to play a significant role in the scheme -- as rusher and receiver -- while sharing time with running backs Fred Jackson and Marshawn Lynch, who split the starting duties last season.
The ACC's player of the year last season finished his four-year college career with 51 touchdowns, including 32 rushing. Listed at 5-foot-11 and 196 pounds, Spiller has dynamic speed and is unafraid to run up the middle.
Because of his versatility, Spiller has drawn comparisons to Reggie Bush of the New Orleans Saints. Spiller and Bush are the only two players to finish their college careers with 3,000 yards rushing, 1,500 yards in kickoff returns, 1,000 yards receiving and 500 yards in punt returns.
The deal comes a day after Spiller celebrated his 23rd birthday and after the player hinted on his Twitter account Thursday that a contract might be in the works. Referring to his birthday, he wrote: "I gt feelin it's going be special."
After practice Friday morning, quarterback Trent Edwards was pleased to learn of the news.
"I'm excited, honestly, because I feel like he's a big impact player on our offense and our team," Edwards said. "The potential is all there. He's got all the talent in the world, and it's just a matter of us blocking for him."
Jackson said he's remained in contact with Spiller through contract talks, and expects the player to be up to speed for practice.
"He's been in his playbook and all the notes he took, so I expect him to come right in and get in the mix of things," Jackson said.
Jackson had a career-best and team-leading 1,062 yards rushing last season, and has no trouble giving up some playing time or with the team's decision to draft Spiller.
"It's a business. They do what they've got to do and I've got to do what I've got to do," Jackson said. "The reps I do get, I'm going to take care of them and continue to try and make plays when I get out there."
The Bills have two other running backs on their roster, Joique Bell and Chad Simpson, both rookie undrafted free agents.
General manager Buddy Nix, Gailey and Bills owner Ralph Wilson all praised Spiller in the days after he was drafted.
Wilson referred to Spiller as "one of the best players in the country," and someone who could provide a spark to what he called a "dull team." Buffalo's offense has ranked 25th or worst in yards the past seven seasons.
Information from The Associated Press was used in this report.
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