The deal, which is pending a physical, is for $27 million over three years with a fourth-year option, according to FanRag Sports.
The 31-year-old McGee, who was 0-2 with three saves and a 3.61 ERA in 62 relief appearances last season, will join Bryan Shaw, who agreed to a three-year deal Tuesday, in the Rockies bullpen.
McGee had a team-high 15 saves in 2016, then was a key setup arm out of the team's revamped bullpen last season as Colorado earned its first postseason appearance since 2009.
He turned in his best season three years ago, when he went 5-2 with a 1.89 ERA and 19 saves in 73 appearances for Tampa Bay.
McGee is 23-16 with a 3.15 ERA and 44 saves in 416 relief appearances over seven seasons with Colorado and the Rays.