The deal is for one year for roughly $4 million and contains a second-year vesting option.
He found another gear after joining the Rays in May, posting a 2.18 ERA and 1.12 WHIP in 45.1 innings.
During his 12-year career, which has also included stints with the Philadelphia Phillies, Texas Rangers, Oakland Athletics, Kansas City Royals, Arizona Diamondbacks, and Boston Red Sox, Diekman has a 3.82 ERA and 1.39 WHIP with a strikeout rate of 11.4 per nine.
In addition to stifling left-handed hitters, Diekman has fared very well against right-handers during his career, holding them to a .209/.327/.345 triple slash.
Last season, Diekman was even better against righties than lefties, limiting them to a minuscule .151/.289/.235 line.
Diekman will be joining a relief corps led by Edwin Diaz, a dominant end-of-game force who should be back at full strength after missing the entire 2023 season due to a freak knee injury.
In addition to Diaz are the recently re-signed Adam Ottavino, Brooks Raley, and Drew Smith, who should all figure prominently. The club also came to terms on a one-year deal with hard-throwing right-hander Shintaro Fujinami on Friday.
Before signing Ottavino, Diekman and Fujinami, the Mets had made a plethora of bullpen additions this offseason, but none had been slam dunks.
Jorge Lopez was the boldest one, and he has tantalizing stuff. But he had a 5.95 ERA in 2023, has a career ERA of 5.51 (with a 1.48 WHIP), and has put together just one big league season out of eight that wasn't below average -- an All-Star campaign for the Orioles in 2022 when he had a 2.54 ERA.