Ramirez, 37, was hitting .247/.295/.430 with 11 home runs and 42 RBIs for the Brewers. After a slow start, Ramirez has hit .352 with two homers in July.
The Pirates will make a corresponding move on their active roster after Ramirez joins the team. That likely won't occur until Saturday. To clear a spot on their 40-man roster, the Pirates designated utility man Steve Lombardozzi for assignment.
Ramirez will fill a major hole in the Pirates' infield. Utility man Josh Harrison (torn ligament in left thumb) and shortstop Jordy Mercer (left knee sprain) could be sidelined until early September.