Ned Grabavoy's 78th-minute goal was enough to hand Salt Lake a 1-0 win in front of 20,496 spectators at Rio Tinto Stadium.
Salt Lake are the only undefeated team remaining in the competition with three wins and four draws from seven games this season in the Western Conference.
Jeff Cassar's team have collected 13 points, three less than pace-setters Dallas, who accounted for Toronto 2-1.
The Timbers, meanwhile, are still searching for their first victory of the campaign.
The Timbers, who have never beaten Salt Lake at Rio Tinto Stadium, were on the back foot for most of the opening 45 minutes but managed to keep the scores level at half time.
Portland had two opportunities two take the lead courtesy of Maximiliano Urruti but he was unable to convert.
And Portland's lack of killer-instinct came back to haunt them as Grabavoy struck 12 minutes from time to maintain Salt Lake's good start to the season.
Defending champions Sporting Kansas City were big winners, easily disposing of the Montreal Impact 4-0.
A brace from English striker Dominic Dwyer and goals from Aurelien Collin and Calum Mallace (own goal) inspired Peter Vermes' men to a comprehensive victory over the Impact, who sit bottom of the Eastern Conference without a win.
As for Sporting KC, they are top of the standings in the East on 11 points from their six games.
The Seattle Sounders were the only other team to win over the weekend, beating 10-man Chivas USA 2-1.
Erick Torres put Chivas ahead after five minutes but goals from Lamar Neagle and Obafemi Martins set up a well deserved win for the visitors.
In other results, the Vancouver Whitecaps and the LA Galaxy shared the spoils after their 2-2 draw.
Hector Jimenez struck in the 90th minute as 10-man Columbus Crew salvaged a 1-1 draw at home to DC United.
Meanwhile, games between the Colorado Rapids and San Jose Earthquakes, and the Philadelphia Union and Houston Dynamo both finished scoreless.
Chicago Fire and the New England Revolution also played out a 1-1 stalemate.
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