Firefighters said a bolt of lightning hit the water and the electrical current then traveled, hitting swimmers and surfers in and out of the water near the 3500 block of Ocean Front Walk around 2:30 p.m.
Thirteen people, including a 15-year-old, were examined at the scene, and eight patients were hospitalized after being struck near the Driftwood lifeguard tower, city fire spokeswoman Katherine Main said.
Five others were injured, but did not require hospitalization, she said.
The man who died was swimming in the water and disappeared under the waves after the lightning strike. He was pulled from the water 90 minutes later, given CPR and transported to Marina Del Rey Hospital in critical condition, where he succumbed to his injuries. His identity was not immediately released.
The other man who was critically injured was a surfer. He was transported to Ronald Reagan UCLA Medical Center in Westwood.
Los Angeles County lifeguards say it doesn't appear either the deceased swimmer or surfer took a direct hit, but the deadly surge of electricity knocked both of them unconscious.
Two people were transported with minor injuries to Saint John's Health Center in Santa Monica. Officials say the majority of the victims were mainly shaken up and were expected to recover.
Los Angeles County Fire Lifeguard Division Capt. Danny Douglas said he had never seen anything like this before, and that authorities were still trying to determine what exactly happened.
"We don't have any idea whether (lightning) hit the water, hit the land or hit some buildings," Douglas said. "We just have reports of one and then some people are saying multiple lightning strikes."
Los Angeles Mayor Eric Garcetti released the following statement on the lightning strike Sunday night: "Amy's and my prayers are with the victims of the lightning strike today at Venice Beach. This tragedy reminds us that we can take nothing for granted or underestimate the power of nature."
Those who were at the scene and saw the lightning strike say it was a horrifying experience. Others said they felt the electricity as it surged through the water.
"Out of nowhere, we heard like a thunderstorm," witness Jason Alas said. "It was like really, really loud."
Stuart Acher was struck by lightning while playing volleyball.
"We went about our game and then all of a sudden, there was a big flash of light and a boom, and it felt like someone punched me in the back of my head," Acher said. "It went down my whole side of my right body, and my calves sort of locked up, and I fell over. And I looked up and everybody else was, you know, falling over."
Acher was treated by paramedics at the scene and released. He felt all right and went back to playing volleyball.
A man, who only wanted to be identified as Paul, was knocked unconscious by the lightning strike. He left St. John's Health Center Sunday night after being treated and thanks his partner, Jose Mendoza, and another friend for saving his life.
"I was in the water, I was boogie boarding and the next thing I know I was trying to get my head above the water for air," Paul said.
Man struck by lightning on golf course on Catalina Island
At least three lightning strikes sparked two brush fires in the Santa Catalina Island area around 12:30 p.m. Sunday. The fires were quickly doused.
A 57-year-old man was struck by lightning on the golf course, Avalon Sheriff's Station deputies said. He was treated and transported to Catalina Island Medical Center in stable condition.
Lightning and fires caused downed power lines and minor flooding in the area.
Car catches fire after lightning strikes home in Redondo Beach
A car caught on fire after lightning struck a home in Redondo Beach Sunday, knocking wires down.
The incident occurred in the 1600 block of Haynes Lane.
Three to four homes were damaged. No one was injured.
The unusual weather came from monsoon moisture that brought a line of brief but fierce afternoon thunderstorms to the region. The storm dissipated as it moved northwest, up the coast.
This was the first death attributed to lightning in California this year.
ABC News, The Associated Press and CNS contributed to this report.