Captain Ahab is a fictional character and one of the protagonists in Herman Melville's Moby-Dick (1851). He is the monomaniacal captain of the whaling ship Pequod. On a previous voyage, the white whale Moby Dick bit off Ahab's leg and he now wears a prosthetic leg made out of ivory. The whaling voyage of the Pequod ends up as a hunt for revenge on the whale, as Ahab forces the crew members to support his fanatical mission. When Moby Dick is finally sighted, Ahab's hatred robs him of all caution, and the whale drags him to his death beneath the sea and sinks the Pequod.

Melville biographer Andrew Delbanco calls Ahab "a brilliant personification of the very essence of fanaticism". Scholar F. O. Matthiessen calls attention to the fact that Ahab is called an "ungodly god-like man". Ahab's "tragedy is that of an unregenerate will" whose "burning mind is barred out from the exuberance of love" and argues that he "remains damned". Writer D. H. Lawrence felt little sympathy for Ahab and found that the whale should have "torn off both his legs, and a bit more besides".

The character of Ahab was created under the influence of Samuel Taylor Coleridge's lecture on Hamlet and figures in biblical and classical literature such as Shakespeare and Milton. His prosthesis, for instance, has been taken for an allusion to the Oedipus myth.

Ahab is firmly established in popular culture by cartoons, comic books, films and plays. Most famously, he provided J. M. Barrie with the model for his Captain Hook character, who is obsessed with not a whale but a crocodile.

Ahab was named by his insane, widowed mother, who died when he was twelve months old. The etymology of the name Ahab derives from the Hebrew, meaning "father's brother" as cited in Strong's Concordance no. 256, and his name is an auspicious allusion to the Biblical character of the same name, known for his wickedness and idolatry. At age 18, Ahab first took to sea as a harpooner. Less than three voyages prior to the start of the novel, Ahab married a young woman, with whom he had a young son.

Captain Peleg, one of the co-owners of the Pequod, sailed as mate under Ahab on a previous voyage in which a typhoon near Japan broke all three of her masts and flung them overboard. While the crew feared that the ship would sink, Ahab and Peleg instead concentrated on saving all hands and rigging temporary masts in order to reach the nearest port and make repairs.

Before the ship sails from Nantucket, Ishmael encounters a man named Elijah, who tells him about some of Ahab's past deeds. According to Elijah, Ahab once lay near death for three days and nights near Cape Horn, took part in a deadly battle against Spanish forces before an altar in Santa, and spat into its silver chalice. Ahab lost his leg during his most recent whaling voyage, leaving him with a grim disposition and a strong desire for revenge against Moby Dick.

In addition to the prosthetic leg, Ahab has a mark that runs down one side of his face and neck: “Threading its way out from among his grey hairs, and continuing right down one side of his tawny scorched face and neck, till it disappeared in his clothing, you saw a slender rod-like mark, lividly whitish. It resembled that perpendicular seam sometimes made in the straight, lofty trunk of a great tree, when the upper lightning tearingly darts down it...leaving the tree still greenly alive, but branded.” –(Moby-Dick, p. 129.) The mark and its origins – whether a birthmark, the scar from a wound, or otherwise – are rarely mentioned or discussed. Ahab's leg includes a small flat patch that he uses as a slate for making navigational calculations. The deck planks of the Pequod have been bored with shallow holes, the same diameter as the lower end, to allow him to steady himself against the motion of the ship. While at sea, he turns to the ship's carpenter and blacksmith to fashion a replacement leg and fittings after damaging the one he wears.