Throughout the 2017 MLS season, Frei confirmed himself as one of the league's best goalkeepers once again: he started 33 games throughout the regular season, making 84 saves over the course of the year for a 69.4 per cent save percentage, and averaging 1.09 goals against per game, while also managing to keep 13 clean sheets, more than any other goalkeeper. In the Playoffs, Frei only made four saves en route to the MLS Cup final, but kept four consecutive clean sheets, as Seattle reached the final without conceding a goal. On 9 December 2017, he started in the 2017 MLS Cup Final against his former club Toronto FC at BMO Field, a rematch of the previous season's final; Toronto won the match 2–0 to complete an unprecedented treble of the MLS Cup, the Supporters' Shield, and the Canadian Championship, but Frei drew praise in the media for his performance throughout the match, following a string of impressive saves to keep the score level at half time, and made nine stops overall within the first hour of play. He finished in third place in the MLS Goalkeeper of the Year Award, behind Tim Melia and Andre Blake. The following year, he placed second in the award, behind only Zack Steffen.