In a battle between the titleholders and last season's Supporters' Shield winners LAFC, the Sounders came out on top thanks to Morris on Sunday.
The Sounders broke the deadlock in the 11th minute via Raul Ruidiaz's stunning long-range effort at CenturyLink Field in Seattle.
Morris made it 3-0 after scoring twice within a 61-second span to shock LAFC – the Sounders forward outpacing two defenders and dodging the onrushing Kenneth Vermeer for his first of the night.
The American then cushioned home a volley from a Joao Paulo cross in the 49th minute.
Diego Rossi pulled one goal back for LAFC on the hour mark, but it was not enough away from home.
Seattle are second in the Western Conference and two points behind leaders Sporting Kansas City, while LAFC are seventh.
Elsewhere, Nashville edged fellow expansion franchise Inter Miami 1-0 thanks to Anibal Godoy's superb strike.
Godoy curled a shot into the top corner of the net from the edge of the area in the 53rd minute to sink Inter.
Nashville are 10th in the Eastern Conference with two wins from eight games, while David Beckham's Inter are bottom with three points.