Authorities said the man was among several people gathered near the jetty, when a large wave knocked a 49-year-old male and 14-year-old boy into the breakwater around 6:30 a.m.
The man jumped in to help, according to a spokesman for the Port of Los Angeles, while others attempted to help pull the victims in.
When rescue crews arrived on scene, they found the man unconscious and not breathing. He later succumbed to his injuries at Providence Little Company of Mary Medical Center in Torrance. Officials have not released his name.
The two victims who were initially knocked into the breakwater were pulled out by others and transported to a local hospital in fair condition. They have since been released.
A port spokesman said the two surviving victims were allegedly fishing in a restricted area. The jetty is shaped like a wall with vertical rock sides.
City News Service contributed to this report.