Monteiro scored the tiebreaking goal in the 80th minute for the Earthquakes (3-5-4, 13 points).
The Earthquakes scored the decisive goal after a pass was broken up near the net. San Jose received a favorable bounce as the wide-open Monteiro pounced on the ball and sent a right-footed shot into the net for his third goal of the season.
The Earthquakes had a 17-9 advantage in shots. The Timbers had a 5-4 edge in shots on target while falling to 1-4-1 over their past six MLS matches.
Portland's lone win during the stretch was when it set a franchise single-game record for goals in a 7-2 rout of Sporting Kansas City on Saturday.
The Timbers struck first in the 18th minute when Marvin Loria took the corner kick and sent the ball into the pack in front of the net. Paredes put his head on the ball and knocked it into the left corner of the net for his second goal of the season.
The Earthquakes knotted the score 13 minutes later as Jeremy Ebobisse anticipated Yueill would come forward for his cross. Yueill reached the ball on the run and sent a diving header that hit the right arm of Ivacic on its way into the net for his second tally of the campaign.
San Jose moved ahead in the 44th minute behind a swift counterattack led by Cade Cowell. As he closed in, Cowell fed Monteiro, who sent a shot into the right corner of the net.
The Timbers tied the score in the 56th minute when Eryk Williamson took the corner kick and booted it in front of the net. Tuiloma was in good position and sent a header in for his fourth goal of the season.