The attacks took place in the coastal city of Half Moon Bay. Some of the victims were Chinese-American.
Suspect Chunli Zhao, 66, was arrested after driving to a police station.
It comes as the state mourns the deaths of 11 people in Monterey Park - about six hours south-east of Half Moon Bay - during Lunar New Year celebrations.
And just over a week ago, six people including a teenage mother and baby were killed at a property in Goshen, central California.
California Governor Gavin Newsom tweeted that he was at a hospital meeting with victims of the Monterey Park mass shooting when he was pulled away to be briefed about the second attack, describing it as "tragedy upon tragedy".
The latest bloodshed to hit the state took place at two farms around the Half Moon Bay community in what officials have described as instances of workplace violence.
Four victims were discovered at a mushroom farm, while the other three were later found at a nearby trucking business. An eighth victim who was transported to hospital is out of surgery and in a stable condition, San Mateo County Sheriff Christina Corpus said at a news conference on Tuesday.
The victims were seven adult men and one woman of Hispanic and Asian descent, Sheriff Corpus said. All of them were agricultural workers, Half Moon Bay Mayor Deborah Penrose told CNN on Tuesday.
The suspect was a resident of Half Moon Bay and employed at the Mountain Mushroom Farm, Sheriff Corpus said.
"The only connection between the suspects and the victims is that they may have been co-workers", Sheriff Corpus said. "All of the evidence we have points to this being an instance of workplace violence."
Zhao is scheduled to be arraigned Wednesday in Redwood City, California, according to local US media outlets.