The Mariners designated Brennan for assignment Wednesday.
Seattle moved Brennan off the 40-man roster to clear a spot for catcher Jacob Nottingham, whom the organization claimed off waivers from the Brewers. The 29-year-old Brennan appeared in 49 games out of the Seattle bullpen between 2019 and 2020, logging a 4.45 ERA and 54:29 K:BB in 54.2 innings.