Temperatures have been particularly fierce in desert areas of the region, easing by only a few degrees at night.
The 25 victims range from 26 to 87 years old. The deaths are being treated as heat-related and autopsies will be carried out along with toxicology tests, coroners said Wednesday.
Weeks are needed to complete the tests, they added.
A 26-year-old was found dead an hour after he separated from his friends while out riding his dirt bike Saturday.
The heat wave also killed an old couple who refused to switch on their air conditioner at the weekend in an attempt to save $50 by keeping the machine off, the Los Angeles Times reported.
The deaths were reported in four counties in Southern California.
Meanwhile, the authorities struggled to fix heat wave-related power shortages that left up to 64,000 homes without electricity.
"We felt like we were being tortured," said Matthew Lorenzen, 28, who suffered power outages several times Saturday, Sunday and Monday. "It was just horrible - lots of still, hot nights."