The veteran journeyman, whose chronic knee condition forced him to retire in May -- only two months after he signed a one-year contract with the Jets -- passed a physical and was removed from the reserve-retired list.
By rule, the Jets have until Oct. 21 to decide whether to activate him to the 53-man roster.
Currently, Garrard has a roster exemption. He will attend meetings and practice with the team.
The move comes two days after the Jets announced season-ending surgery for former starter Mark Sanchez. They already have three quarterbacks on the roster -- rookie Geno Smith, Matt Simms and veteran Brady Quinn, whose roster spot is in jeopardy with Garrard now in the picture.
Garrard spoke to reporters wearing a black T-shirt that read: "Some Things Just Won't Retire."
"I didn't want to turn 50 and be that guy that looks back and says, 'What if? What if I just called somebody?'" the 35-year-old quarterback said after his first practice. "I'm glad I did."
Garrard took to Twitter Thursday to announce his comeback:
I love being a JET again!! pic.twitter.com/bymCpYDLyW
— David Garrard (@davidgarrard9) October 10, 2013
Garrard said he called general manager John Idzik on Sunday as he watched games on TV, asking for another chance. He thought to himself, "I need to be somewhere. There's no reason for me to be sitting at home."
If the Jets didn't want him, he would've requested his release from the reserve list, allowing him to speak to other teams.
The former longtime Jacksonville Jaguars starter said he's convinced his balky knee can handle the wear and tear. How does he know? His wife is the quarterback on a flag-football team, and he ran routes for her in practice.
"That told me I could do it, because how many quarterbacks can run routes?" Garrard said.
Coach Rex Ryan said Garrard is on a two-game trial, adding: "There haven't been any guarantees or promises made."
Garrard signed a one-year, $1.1 million contract in March, which included a $100,000 signing bonus. At the time, the Jets felt so good about him that, privately, they thought he could emerge as the opening-day starter. If not, they envisioned him as the ideal mentor for Smith, who was drafted one month later.
But Garrard's surgically repaired knee was in such bad shape that he couldn't make it out of OTAs.
Garrard said he took the summer off and started working out with his wife. He recently ran three miles on concrete with no knee swelling, reinforcing his belief he could make a comeback.
The Jets see value in Garrard because of his "unbelievable wealth of knowledge and experience," according to Ryan. Garrard has no illusions about taking the starting job from Smith, and he's on board with the idea of serving as a mentor.
There was some thought that Quinn, 28, could fill that role, but Ryan said he sees him as a "young quarterback." Quinn said he had "no reaction" to Garrard's return, adding: "This doesn't change anything. This is a week-to-week business, especially right now."
Garrard played nine seasons for the Jaguars, but he hasn't played in a regular-season game since 2010. He sat out the 2011 season after back surgery, and a comeback attempted last year with the Miami Dolphins ended when he underwent knee surgery in the preseason.