Whale-watching vessels reported seeing 20 to 40 of the marine mammals, also known as orcas, swimming north up the coast from Dana Point to Long Beach.
It's not uncommon to see killer whales in Southern California waters in search of sea lions and other prey, but such large numbers are unusual, marine scientists said.
One tour boat, the Dana Pride, was on a chartered whale-watching trip for elementary school children when it came across a pod of 20 killer whales.
"They got the time of their lives seeing all those orcas," Donna Kalez, general manager of Dana Wharf Sportfishing & Whale Watching, told the Los Angeles Times.
Another boat encountered as many as 40 killer whales spread out over a large area.
"It was an incredible encounter," Dave Anderson of Captain Dave's Dolphin and Whale Safari said. "They were coming over to our boat, swimming up next to the boat, turning upside-down and doing a lot of tail-slapping."
The orcas may be from an "offshore" population based in British Columbia that travel in large numbers and prefer to feed on sharks, experts said.
The offshore orcas were last seen close to the Southern California shore in 2005.
"We don't really know where they go or where they come from," Kera Mathes, whale research biologist with the Aquarium of the Pacific, said. "We just see them every couple of years."