A three-time All-Star with 126 career saves, Hand struggled badly with the Blue Jays after arriving from Washington in a July 29 trade. The left-hander went 0-2 with a 7.27 ERA in 11 appearances for Toronto and was designated for assignment Tuesday.
New York was interested in signing Hand as a free agent last offseason before he inked a $10.5 million, one-year contract with division-rival Washington. He pitched in 41 games for the Nationals, going 5-5 with a 3.59 ERA and 21 saves.
The 31-year-old Hand was shipped to Toronto for rookie catcher Riley Adams as part of a Washington teardown leading up to the trade deadline.
Hand gave up 10 runs (seven earned) and 13 hits -- including three homers -- over 8 2/3 innings with the Blue Jays.
"This is a high-leverage guy. We understand that he's been going through some struggles lately, but this is going to be a refresh for him," Mets manager Luis Rojas said. "He pitched really well against us this year, a guy that's been used as a closer with different teams now, so I think it's an upgrade for us. It's a great acquisition by our front office, sending a message to us, the team and to the whole Mets nation out there that we're fighting for this and we're upgrading our team."
New York was third in the NL East, five games behind first-place Atlanta, entering Thursday night's series finale against the Marlins.
"There's some division rivals and some teams that we're going to see in the remainder of our schedule that have pretty important left-handed batters in the middle of their order, and this guy can help us shut them down," Rojas added.
Hand is 31-50 with a 3.70 ERA during his 11-year major league career. He was an All-Star every year from 2017 to 2019 with San Diego and Cleveland.
He is expected to join the Mets this weekend in Washington, possibly as soon as Friday.
To make room on the roster, right-hander Geoff Hartlieb was designated for assignment.
The Mets were rained out Wednesday, when roster limits expanded to 28 players. The team still made a couple of moves, recalling outfielders Albert Almora Jr. and Khalil Lee from Triple-A Syracuse.
Lee didn't stick around long, as he was optioned back to Syracuse prior to Thursday's game when New York recalled right-handed reliever Yennsy Diaz from Triple-A.