MLB Network first reported the news.
Pillar, 32, finished the pandemic-shortened 2020 season with the Colorado Rockies after a trade-deadline deal in August from the Boston Red Sox. He batted .308 with two home runs and 13 RBIs in 24 games with the Rockies.
On Tuesday, the Mets assigned right-hander Corey Oswalt outright to Triple-A Syracuse and released right-hander Brad Bach. The two moves created roster spots for infielder Jonathan Villar, who finalized a $3.55 million, one-year contract last Thursday, and outfielder Albert Almora, who completed a $1.25 million, one-year contract last Wednesday.
New York will have to make another move when it finalizes its contract with Pillar.
Pillar was originally selected by the Toronto Blue Jays in the 32nd round of the 2011 draft. He is a career .262 hitter and has five seasons with double-digit steals, including 25 in 2015.
Pillar is joining his fifth team. He was traded in April 2019 from the Blue Jays to the San Francisco Giants, and he signed a deal with Boston in February 2020.
The Associated Press contributed to this report.