Sounders FC went to Montreal, grabbed a lead, but failed to hold on to it. Seattle loses to the Impact, 2-1 on Wednesday evening at Stade Saputo.
Víctor Rodríguez scored Seattle's lone goal, his second of the season.
But a late brace by Saphir Taïder gave Montreal a come-from-behind victory.
The Sounders broke the scoreless match in the 64th minute. Harry Shipp was fouled inside the area. The Spaniard converted his spot-kick to the upper right corner past Montreal goalkeeper Evan Bush.
Ten minutes later, Montreal earns a PK and Taïder levels the match at 1-1.
Minutes later, Taïder completed his brace to give Montreal the lead for good, redirecting a cross from Omar Browne for his team-leading seventh goal of the season.
Due to international callups and injuries, Sounders FC had six substitutes available for tonight's match, including two goalkeepers. Midfielder Danny Leyva made his MLS debut as a substitute in the 82nd minute, becoming the third-youngest player in league history to appear in a match. At 16 years and 31 days, Leyva trails only Freddy Adu (14 years, 306 days) and Alphonso Davies (15 years, 257 days).