Emergency crews and state troopers responded to the scene at about 7 a.m. after receiving reports of a serious crash.
"It's obviously a pretty touchy situation and cars are backing up quickly," said Mike Allende of the state Department of Transportation said in the moments after the crash.
The backup stretched south nearly to Federal Way as of 8:05 a.m.
Four lanes were blocked initially. Later, crews were able to open up three lanes to traffic, then all lanes were reopened. The backup had shrunk to about a mile or less by 8:50 a.m.
There were no reports of serious injuries from the crash.
It was the second time in two days that a crash has snarled the south end commute. On Tuesday, a semi truck rolled over on I-405 near Renton, closing down the southbound lanes of the freeway for three hours.