The deal is for five years and worth $36.25 million with $15 million guaranteed, a source told ESPN NFL Insider Adam Schefter.
It is easily the Jets' biggest free-agent signing since John Idzik became the general manager 14 months ago as Decker was considered by some the top receiver on the free-agent market.
Decker arrived in New Jersey late Tuesday night and spent Wednesday meeting with team officials in Florham Park. He capped Wednesday by dining out with Rex Ryan and other team personnel.
"I'm very thankful for my four years in Denver," Decker told the Denver Post late Wednesday night. "I think it's one of the greatest cities to play in. Unfortunately, I never got an offer from the Broncos. Denver will always be a part of me but I'm excited about New York being my new community and starting a new chapter in my life. On top of all that, my wife's expecting our baby any second."
The Jets are trying to rebuild at wide receiver after finishing 31st in passing offense, hoping to accelerate Geno Smith's growth by surrounding the second-year quarterback with better weapons. On Monday, they released their most accomplished wideout, Santonio Holmes, in a salary dump.
Decker, who turns 27 Saturday, is coming off back-to-back, 1,000-yard seasons. He recorded career highs last season with 87 receptions and 1,288 yards, contributing to the most prolific passing offense in league history.
But some scouts believe his gaudy numbers were a product of playing with Peyton Manning, questioning whether he could be successful as a No. 1 receiver. He was a non-factor against the Seattle Seahawks in Super Bowl XLVIII.
Decker was also a holdover from one of the two Josh McDaniels' draft classes (he was a third-round pick in 2010, the same year the team took Demaryius Thomas in the first round).
He was a bit of a medical concern when he entered the NFL after foot surgery in his final season at Minnesota. But he put up back-to-back 1,000-yard seasons with Manning at quarterback.
He usually played in one of the outside spots, but the Broncos used him in the slot at times as well. Decker was also used in a punt return role at times in his career.
Overall, he struggled at times against some of the more physical defensive backs he faced -- he had just one catch in the Broncos' Super Bowl loss -- but he was effective in the red zone once Manning was behind center.
He had 11 touchdowns this past season to go with 13 in 2012, which put him on a short list of players to top double-digit touchdowns in each of the last two years.
Decker said he's not concerned about the prospect of going from Manning to the unproven Smith.
"When you go from a Hall-of-Fame quarterback, no one else -- no matter who it is -- can be on the same level," he told the Post. "Talking to the offensive coordinator Marty [Mornhinweg], who has had a good track record in San Francisco and Philadelphia, he really believes in this kid. When you look at the statistics, yeah, he got off to a slow start, but he was hurt early on and he finished the season strong. He improved all season long in a tough market. That's pretty impressive."
ESPN.com Broncos reporter Jeff Legwold contributed to this report.