Seattle (8-3-3, 27 points) is in first place, two points ahead of Chicago, with six matches to play.
Sunday, Beverly Yanez scored two second-half goals within a five-minute span to put the game away. Jessica Fishlock scored the opening goal in the 38th minute.
Seattle dominated possession and pressure throughout the match, outshooting Portland 16-5.
Germany goalkeeper Nadine Angerer started for Portland for the first time since the World Cup. She had six saves.
The Thorns face a quick turnaround for their next match on Wednesday at Western New York.