The second-year quarterback will start the remaining four games, replacing the embattled Ryan Fitzpatrick in a long-anticipated move.
Coach Todd Bowles made the announcement immediately after Monday night's 41-10 loss to the Indianapolis Colts at MetLife Stadium.
Bowles said it was decided before the game, win or lose, that there would be a quarterback change. The Jets (3-9), losers of four straight, were officially eliminated from playoff contention with the loss.
"The plan was for us to start him the last four games anyway, so we just started a half early," said Bowles, who benched Fitzpatrick at halftime. "We could have won 45-0, and he was going to start the last four games. That's the slot we had penciled in for him. That's a good time to take a look at him."
The move was handled clumsily by the team. Fitzpatrick was informed of his demotion moments before he stepped to the interview podium after the game. Same for Petty. The players in the locker room had no idea of the change until informed by reporters.
"I didn't get it done," said Fitzpatrick, who might have played his last down as a Jet. He will be a free agent, and there's little chance of him returning to the team.
Bowles said he isn't sure if the No. 2 quarterback will be Fitzpatrick or rookie Christian Hackenberg, who has yet to dress for a game. But the coach made it clear that, barring injury, Petty will finish the season.
"It's a great opportunity for me, obviously," said Petty, who has had one career start. "I didn't want it to be this way. I'd much rather have us in the same situation we were in last year, trying to make the playoffs. But what I do know is opportunities are few and far between in this league, and you have to capitalize on any chances you get. I'm going to work my ass off."
Bowles' decision should appease the fan base, which had been clamoring for Petty. He was a 2015 fourth-round pick out of Baylor, and he flashed potential in the preseason.
On Monday, he struggled, completing 11 of 25 passes for 135 yards, one touchdown and two interceptions. He was put into a tough situation, as the Jets trailed 24-3 at halftime.
"He was against the odds," Bowles said. "He threw a couple of good balls. He threw some bad balls."
So did Fitzpatrick, who fell to 2-8 as the Jets' starter. He was 5-for-12 for 81 yards, with an interception. He was disappointed with the benching but handled it well.
"In my situation, I really don't care about the big picture," he said. "My situation is I just want to be out there playing, but I didn't play well enough."