Saitama (サイタマ, Saitama) is the main protagonist of the series and the titular One-Punch Man. He is the most powerful being to exist in the series. Saitama faces a self-imposed existential crisis, as he is now too powerful to gain any thrill from battle.

Initially just a hero for fun, Saitama later registers to be a professional hero for the Hero Association upon realizing that nobody recognizes him as a hero, and defends his home in Z-City from monsters, villains, and other threats. Under the Hero Association, he is assigned the hero name Caped Baldy (ハゲマント, Hagemanto; Viz: Bald Cape) and is currently B-Class Rank 7.

Saitama is an ordinary-looking bald man with a thin but well-built physique and is of average height and weight. He used to have a full head of short black hair but lost it due to the intensity of his hero training regimen.

Saitama is usually deliberately drawn in a more simplistic style than the other characters, similar to the style of the original webcomic, with an elliptical head and simple facial features. When drawn in a more serious style, Saitama is revealed to have sharp facial features, fearsome eyes, and chiseled musculature. His posture changes as well, with a slack posture and sloping shoulders when lax, and a straightened posture and squared shoulders when serious.