Marcum, 31, will get a one-year deal contingent on passing a physical, a major league source confirmed. CBSSports and Fox reported the deal earlier Thursday.
He would fill the void left after NL Cy Young Award winner R.A. Dickey was traded to the Toronto Blue Jays and join Johan Santana, Matt Harvey, Jonathon Niese and Dillon Gee in the Mets' rotation. The addition of Marcum also likely precludes the Mets from re-signing free-agent pitcher Chris Young.
Marcum went 7-4 with a 3.70 ERA in 21 starts last season for the Milwaukee Brewers.