Stratton was 0-2 with an 8.59 ERA in five starts and two relief appearances for the Angels, then was designated for assignment on May 7. He attended spring training with San Francisco and was acquired by the Angels on March 26 for pitcher Williams Jerez.
Stratton won a career-high 10 games for the Giants in 2018. He is 15-16 with one save and a 5.16 ERA in 55 big league appearances.
Pittsburgh transferred right-hander Nick Burdi (right biceps/elbow pain) to the 60-day injured list on Saturday to make room for Burdi on the 40-man roster.