Officers responded to reports of the collision near the intersection of Third and King streets around 9:37 a.m., police said.
Both the woman and the cement truck were heading westbound on King Street when, for unknown reasons, they collided, Sgt. Michael Andraychak said.
The driver stopped immediately after the crash and is cooperating with the police investigation, he said.
The woman was taken to a hospital with life-threatening injuries and was pronounced dead later that morning.
Investigators are attempting to determine how the crash happened. Alcohol and drugs do not appear to have been a factor, police said.
The fatal collision occurred as thousands of people waited to enter the San Francisco Giants Fan Fest.