The deal was announced by the team, and included Buffalo CEO Russ Brandon posting a photo on his Twitter account of Gilmore signing his contract.
"It's official," Brandon wrote. "Nice way to start the morning."
Listed at 6-foot-1 and 190 pounds, Gilmore was drafted 10th overall after a three-year career at South Carolina, where he started all 40 games and was a two-time All-Southeastern Conference selection. The contract includes an option for fifth year.
In four practices, Gilmore had several pass break ups and at least one interception while showing off his coverage skills. He was both fast on his feet in covering receivers deep, and also showed he's capable of bumping opponents coming off the line of scrimmage.
After the minicamp, Gilmore expected to have the contract completed by the end of this week, and is now happy to get it out of the way.
"I wasn't going to let it be a distraction. I wasn't really worried about it," Gilmore said in a statement released by the team. "I knew it was going to come through. Now I can just focus on football and do what I want to do."
Gilmore was the second cornerback selected in the draft, behind LSU's Morris Claiborne, who was picked sixth by Dallas.
The next step for Gilmore comes on May 29, when he will get an opportunity to line up against veteran receivers once the Bills open a monthlong series of minicamps.
"Oh yeah, I'm ready to see what the vets have going on," Gilmore said last weekend. "I'm going to play with confidence, and I'm going to still do what I know, and go from there."
Gilmore made a good first impression on coach Chan Gailey, who noted both the player's athleticism and confident approach.
Gilmore is the latest addition to a defense that's been rebuilt this offseason, and to be overseen by Dave Wannstedt, who takes over as coordinator after George Edwards was fired in January. The Bills have already addressed improving their pass rush in free agency, when they signed defensive ends Mario Williams and Mark Anderson in March.
The moves were made in a bid to improve a defense that allowed a franchise-worst 5,938 yards last season. Buffalo is coming off a 6-10 finish to miss the playoffs for a 12th straight year.