Police said the crash was reported around 12:45 p.m. on the Embarcadero near Bay Street.
Four people suffered minor injuries in the crash, which slowed traffic around the intersection for more than an hour.
The drivers of both vehicles also had minor injuries and were hospitalized for further observation.
Fire officials say the 18-wheeler turned in front of the street car.
The Muni streetcar is Number 162, San Francisco's first streetcar that debuted in 1914.