The 28-year-old Yastrzemski was originally drafted by the Orioles in 2013 and played 703 games with 2,600 at-bats in the minors. San Francisco traded for him in March, a deal that sent minor league right-hander Tyler Herb to the Orioles.
Mac Willamson was designated for assignment to make room for Yastrzemski. Williamson struck out five times in five at-bats against Atlanta on Thursday, then whiffed three more times in the series opener against Arizona on Friday.
The Giants also placed reliever Trevor Gott on the 10-day injured list with a right forearm strain and recalled right-hander Dereck Rodríguez from Sacramento.