Tomas Lopez heard that a man was in trouble in an unguarded section of Hallandale Beach, north of Miami, and so ran to save him.
However, his bosses said that his actions meant that he broke company rules which could have put other swimmers at risk.
Lopez was then asked to leave his $8.25-an-hour (£5.30) job but he does not regret his decision, adding to CBS television: "I think it's ridiculous, honestly, that a sign is what separates someone from being safe and not safe."
The 21-year-old lifeguard ran quite a distance to help the drowning man but he had already been pulled out of the water by some other beachgoers. His actions did prove useful, however, as he and an off-duty nurse aided the man until paramedics reached the scene.
The man is now said to be in intensive care in hospital.
After hearing that Lopez was fired, at least two of his colleagues have resigned in protest.
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