The lights went out at about 9:30 a.m. in a wide swath from Alki Avenue SW down as far south as White Center and from the Duwamish River to Puget Sound.
By 1:45 p.m., power had been restored to most customers. Only about 750 customers remained without electricity in scattered areas.
Crews expect to restore power to all customers by 3:30 p.m.
A Seattle City Light news release said the outage was caused by a tree branch that fell into power lines near SW Hanford Street and 46th Avenue SW.