Malachi Roland St. John "Buck" Mulligan is a fictional character in James Joyce's 1922 novel Ulysses. He appears most prominently in episode 1 ("Telemachus"), and is the subject of the novel's famous first sentence: "Stately, plump Buck Mulligan came from the stairhead, bearing a bowl of lather on which a mirror and a razor lay crossed."

Buck Mulligan is described as having a "face... equine in its length", a "sullen oval jowl", a "strong wellknit trunk", "light untonsured hair, grained and hued like pale oak", "even white teeth", and "smokeblue mobile eyes." He begins the morning in a yellow dressing-gown; later he dons a distinctive primrose waistcoat and Panama hat. His facial expressions often shift rapidly, and he is prone to sudden, energetic movements.

Mulligan is a medical student with a cynical view of the human condition, which he describes as "a mockery and beastly". Paradoxically, he is also the most consistently cheerful character in Ulysses, and is portrayed as being in constant pursuit of the next opportunity to eat, drink, and make merry. He is widely regarded as a hero for having saved men from drowning, and appears to be well liked by all the characters in the book, with the exception of Simon Dedalus (who dismisses him as a "bastard" and a "contaminated bloody doubledyed ruffian") and, to a lesser extent, Leopold Bloom.

Mulligan is an avid classicist and espouses the belief that Ireland ought to be "Hellenized". His speeches contain a barrage of quotations from poets (notably Swinburne and Whitman), popular songs, and self-composed lines of parody and ribaldry. He also seems to admire the philosophy of Nietzsche, referring to himself as a "hyperborean" and (more facetiously) "the Übermensch." He expresses disdain for the Celtic Revival when in the company of Stephen Dedalus, but is socially active in Dublin's literary circles.

Mulligan's finances appear to be at least partially dependent on the generosity of a wealthy, pious aunt; he is also mentioned as having a father who was a "counter-jumper" (i.e. sales clerk), a mother, and a brother.

Mulligan does not appear as a character in A Portrait of the Artist as a Young Man, but his acquaintance with Stephen Dedalus has been of some substantial duration by the start of Ulysses. The pair share quarters at the Sandycove Martello Tower, whose twelve-pound yearly rent the chronically impecunious Stephen has somehow contrived to pay.

Mulligan's attitude towards Stephen in conversation is both playful and patronising; he alternately teases and compliments Stephen's physical appearance, and refers to him by such epithets as "Kinch" (in evocation of a knife-blade), "Wandering Aengus" (a dual reference to the poetry of W. B. Yeats and to Stephen's demeanor whilst drunk), and "dogsbody". He is frequently generous with Stephen, lending him money and clothing, but also carelessly makes free with Stephen's own possessions and funds, importunities which Stephen seems to accept out of a sense of obligation. Mulligan also injures Stephen with callous remarks about Stephen's late mother and his conduct towards her. Although Stephen's mother has been dead for ten months, Stephen has seemingly never vocalised any grievances to Mulligan concerning these remarks until the opening chapter of Ulysses.

Stephen, meanwhile, has come to regard Mulligan as an antagonist, privately referring to him as "mine enemy." He interprets a request for the Tower key at the end of chapter one as an attempt by Mulligan to "usurp" the Tower from him, and eventually resolves to part company with Mulligan altogether. Stephen also harbours feelings of insecurity about Mulligan's physical courage and fearlessness, traits which Stephen feels that he himself does not possess.