 On November 1, the Blue Jays picked up Happ's $6.7 million option for the 2015 season.On December 3, 2014, Happ was traded to the Seattle Mariners in exchange for outfielder Michael Saunders.Happ made 20 starts and 1 relief appearance for the Mariners, compiling a 4–6 record with a 4.64 ERA and 82 strikeouts.