ESPN NFL Insider Adam Schefter first reported the two sides had reached an agreement. The deal is worth as much as $25 million, ESPN.com's John Clayton reported. Guaranteed money and other contract terms were not immediately available.
"Ecstatic, overjoyed, whatever you want to call it, that's me," said Maybin, who flew from Baltimore to Buffalo to sign the deal. "I've been smiling from ear to ear for the entire day, man, ever since I got off the plane."
"Aaron's happy to be here and ready to go to work," Chafie Fields, Maybin's agent, said.
Maybin is expected to begin practicing on Monday, the team's first session after it returns following Saturday's preseason game at Green Bay. Buffalo has two preseason games left, at Pittsburgh on Aug. 29 and at home against Detroit on Sept. 3.
"There's a lot of catching up that I have to do. But we still have three weeks to go," Maybin said. "I'm really excited about getting back out there, getting back on the field. And I think I'll be fine."
The Bills are counting on Maybin, whom they drafted at No. 11 from Penn State, to shore up a defense that produced only 24 sacks last season -- tied for the fifth-fewest in the NFL.
After sitting out his first year at Penn State as a redshirt freshman, Maybin entered the draft after two college seasons and without a full season as a starter.
Though he played 13 games last season, Maybin didn't take over as a full-time starter until the fourth week. But he led the Big Ten with 12 sacks and was voted a first-team All-American.
Maybin unveiled a new haircut in honor of the deal.
Cut into the right side of his hair were two numbers: 58, the one he'll wear this season; and 78, in a tribute to former Bills defensive end Bruce Smith, who was inducted into the Pro Football Hall of Fame earlier this month. Cut into the left side of his hair was the Bills' logo of a running buffalo.
Maybin laughed when asked whether the logo meant he won't require a helmet.
"Hey man," Maybin said. "I'm a tough guy, but not that tough."
The agreement leaves two unsigned first-round picks: offensive tackle Andre Smith, who was taken at No. 6 by the Cincinnati Bengals, and wide receiver Michael Crabtree, the No. 10 pick of the San Francisco 49ers.
Information from The Associated Press was used in this report.