The switch-hitting Doumit, who batted .247 with 14 homers and 55 RBI in 135 games for the Twins last season, will provide depth at catcher for the National League East champions behind Evan Gattis and Gerald Laird.
The 32-year-old is an effective pinch-hitter, holding a .262 career average coming off the bench.
Doumit can also play outfield and first base, with a combined 105 games at those positions in his career with Pittsburgh and Minnesota.
Over 880 total career games, he sports a .268 average with 99 home runs and 396 RBI.
Going to the Twins is 2011 Braves first-round pick Gilmartin, who is coming off a disappointing 2013 season with Class AAA Gwinnett. The lefty went 3-8 with a 5.74 ERA in 17 starts.