Ray Allen scored 27 of his season-high 42 points in the fourth quarter and overtime.
"I wanted the ball in my hands and I'm sure the whole team wanted the ball in my hands," Allen said. "It was just one of those nights, it worked well for us."
A three-time All-Star, Allen had seven points in the extra period, when Seattle outscored Portland, 15-4. On Friday, he scored 35 points and outdueled Kobe Bryant as the SuperSonics defeated the Los Angeles Lakers, 111-109.
Vladimir Radmanovic added 21 points for Seattle, which has won six of its last eight games.
Randolph led Portland with 28 points and 12 rebounds and Damon Stoudamire scored 22 points. But the Trail Blazers have lost three in a row and fell to 1-12 on the road.
Coming off their most lopsided loss of the season, Sunday's 130-99 setback to Sacramento, the SuperSonics trailed by as many as 23 points in the third quarter.