Justin Stone, 34, was riding his skateboard Wednesday when he hit a bump and was thrown into an intersection, the San Francisco Chronicle reported.
He landed under a passing truck, police spokesman Albie Esparza said.
The truck crushed Stone, the medical examiner said.
Two police officers saw the accident and rushed to help Stone, Esparza said. They administered CPR until paramedics arrived and took him to a hospital where Stone was pronounced dead.
The truck driver was cooperating with police, Esparza said.
"There have been no arrests," he said. "It appears to be nothing more than an unfortunate accident."
A few hours after the accident, a 61-year-old woman was hit by a car while crossing an intersection, police said.
The car that hit the woman had swerved around another car that had stopped to let the woman cross.
The woman was taken to a hospital with life-threatening injuries, Esparza said. Her name has not been released.
"There have been no arrests," police spokesman Sgt. Daryl Fong said. "There were a number of witnesses, and we're still looking into what occurred."