Seattle Fire Department spokesman Kyle Moore said officials aren't sure what caused the semi to topple over onto its side. It did not land on top of any other vehicles.
The driver was able to make it out of the cab. He did not sustain any serious injuries.
The truck was carrying 50 gallons of diesel fuel, and some of that is leaking toward storm drains, Moore said.
"We want to make sure this diesel doesn't make it down - as much as we can - doesn't make it into the storm drains, doesn't get onto people's cars," Moore said. "So we're going to work with (Seattle Public Utilities) to make sure this is all cleaned up before we reopen that roadway."
Spokane Street remains closed at the scene of the accident, by the lower West Seattle bridge.