At Swim-Two-Birds is a novel by Irish novelist Flann O'Brien (one pen-name of Brian O'Nolan) published in 1939. It is widely considered O'Brien's masterpiece and one of the most sophisticated examples of metafiction.

The novel is narrated by a college student who never goes to class. Instead, he spends his time carousing with friends and smoking cigarettes (in bed, while wearing a single suit of clothes). The college student is studying Irish Gaelic. The student begins to write a novel about an Irish novelist who writes only Westerns. The student's translations of Irish legend (both of Finn MacCool and mad King Sweeney), which are satires of the inflated and culturally unaware translations done by Lady Gregory, begin to appear alongside narratives of college life, the story of a very colloquial pookah, and a "novel" about the Western-writer. The author of Westerns is an eccentric who lives alone in an hotel, and he falls in love with his own description of a female character. He then, Zeus-like, summons her to his room and rapes her. The characters in the Western writer's proposed novel, meanwhile, dislike their narrative and give their author drugs to keep him asleep (and therefore not in control of their world). The author's rape results in the birth of a child, whose upbringing is controlled by the pookah -- a child who will eventually write a novel about his novelist. Just at the point of the child writing a novel about his novelist and torturing his author to death, the college student passes his exams, and At Swim-Two-Birds ends. Interlaced with the two interior fictions is the author's college career through a term at school. This narrative is a sort of Rake's Progress , as the young man engages the literary life of Dublin in the 1930's.

At Swim-Two-Birds has been admired by British and Irish authors for decades, with Dylan Thomas offering high praise ("This is just the book to give your sister – if she's a loud, dirty, boozy girl") and Anthony Burgess considering it one of the ninety-nine greatest novels of the first half of the twentieth century. Publication of the novel in the United States has been spotty, and the work has only recently begun to attract a strong following there. American writer Gilbert Sorrentino paid homage to the book with his sprawling 1979 novel Mulligan Stew , itself a novel about the writing of a novel, with characters drawn from other works (including At Swim-Two-Birds) who wreak havoc upon the fictional author and the text itself. More recently, the title of O'Nolan's book has received a punning appropriation by Jamie O'Neill, the author of At Swim, Two Boys .

While O'Nolan wrote other novels, none achieved either the level of complexity or playfulness of At Swim-Two-Birds, and thus his accomplishments have not been as recognized as those of his contemporaries.