The 48-year-old man was last spotted wearing a pair of blue biker shorts, and did not return to shore when he was expected.
The Coast Guard has deployed both a ship and a helicopter from Point Mugu to help in the search.
The agency posted a picture of the man on Twitter but did not identify him by name.
Anyone who might know the man's location is urged to call authorities at (310) 521-3801.
Coast Guard searching for overdue 48-year-old swimmer last seen wearing blue biker shorts near Corona Del Mar. CG crews searching include Sta. LA/LB boat crew, Forward Operating Base Point Mugu helicopter crew and USCGC Blacktip. Anyone with info should call (310) 521-3801 pic.twitter.com/GremYMu82f— USCG Southern California (@USCGSoCal) May 29, 2022