Troopers say a mudslide pushed the tree into fast-moving traffic on two southbound lanes of I-405 near NE 30th Street at about 4 a.m.
The Route 280 bus slammed head-on into the tree's branches and 12-inch trunk just as it was coming down.
Six passengers and the driver were aboard at the time of the crash. All are unhurt, but visibly shaken by the crash. They told KOMO News that the impact was loud and sudden.
The bus' windshield was shattered by the force of the impact. The bus was towed away, and passengers were transferred to another bus.
A pickup was right behind the bus and hit the tree as well. The driver was shaken up but able to drive away.
State Department of Transportation crews are now inspecting the hillside where the tree came down to assess its stability.
All lanes of the freeway have reopened.