The talented but enigmatic Bryant had asked to be traded out of Pittsburgh in September, and Raiders general manager Reggie McKenzie said Thursday that Oakland had been "on this for a while" when asked about the deal.
Bryant told ESPN's Jeremy Fowler that he was "happy" when he heard the news, calling the trade "a blessing."
"I appreciate the Steelers for everything," he said. "Just trying to move on and try to contribute the best way I can."
The Steelers received the 79th overall pick, which the Raiders received earlier Thursday night when they traded down in the first round, allowing the Arizona Cardinals to select quarterback Josh Rosen at No. 10.
"It's like drafting a player," McKenzie said. "I feel like we drafted Martavis Bryant."
Bryant missed the entire 2016 season because of a suspension for violating the NFL's substance-abuse policy but returned to haul in 50 receptions for 603 yards and three touchdowns last season. He told Fowler that his career in Pittsburgh had "its ups and downs."
"I'll always have love for the Steelers," he said. "There was no bitterness. It's just a business situation. [If I wasn't traded] I was prepared to be back and be a good teammate."
Bryant, 26, has one year left on his contract and joins a Raiders wide receiver corps headlined by Amari Cooper.
"He's ideal in what you're looking for in a receiver," McKenzie said. "He can play big, he plays fast, and he makes plays. And we think he can be a dynamic receiver for Derek [Carr]."
Steelers general manager Kevin Colbert said teams called Pittsburgh during the offseason but weren't offering enough to warrant a trade for a player of Bryant's caliber.
"When it got to the third round, we felt it was something very interesting to us," Colbert said. "We wish Martavis nothing but the best."
Bryant wanted out of Pittsburgh and believed there was a chance he would get dealt sometime this offseason. He has been training in Las Vegas and hadn't reported to the team's voluntary workout program.
He was the perfect deep threat for the Steelers' offense but fell out of favor last season when he requested a trade and publicly questioned the ability of teammate JuJu Smith-Schuster, who now becomes the team's clear-cut No. 2 receiver behind Antonio Brown.
ESPN's Paul Gutierrez contributed to this report.