- Randle McMurphy: A rebellious convict sent from a normal prison. He is guilty of battery and gambling. He had also been charged with, but never convicted of, statutory rape. McMurphy is transferred from a prison work farm to the hospital, thinking it will be an easy way to serve out his sentence in comfort. In the end, McMurphy turns violent against Nurse Ratched, costing him his freedom, his health, and his life.
Chief Bromden: The novel's half-Native American narrator has been in the mental hospital since the end of World War II. Bromden pretends to be deaf and mute, and through this guise he becomes privy to many of the ward's dirtiest secrets. As a young man, the Chief was a high school football star, a college student, and a war hero. After seeing his father, a Native American chieftain, humiliated at the hands of the U.S. government and his (white) wife, Chief Bromden descends into clinical depression and begins hallucinating. Soon he is diagnosed with schizophrenia. He believes society is controlled by a large, mechanized system which he calls "The Combine."
- Richard Gray, author of A History of American Literature, said that Bromden "supplies" the novel's "vision". Gray explains that Bromden's "eye" "sees the inner truth" and that Bromden "is an outsider, an innocent eye in a way like Huck Finn, but what he sees is far stranger, far more surreal." Gray explained that Bromden's vision "may not be literally true but it is symbolically so because, to quote Emily Dickinson again, 'Much madness is divinest sense.'"
- Nurse Ratched (also known as "Big Nurse"): The tyrannical head nurse of the mental institution, who exercises near-total control over those in her care, including her subordinates. She will not hesitate to restrict her patients' access to medication, amenities, and basic human necessities if it suits her whims. Her informant is the timid Billy Bibbit, whom she coerces into divulging the unit's secrets by threatening to complain about him to his mother. McMurphy's fun-loving, rebellious presence in Ratched's institution is a constant annoyance, as neither threats nor punishment nor shock therapy will stop him or the patients under his sway. Eventually, after McMurphy nearly chokes her to death in a fit of rage, Nurse Ratched has him lobotomized. However, the damage has already been done, and Nurse Ratched's rule is broken after McMurphy's attack leaves her nearly unable to speak, which renders her unable to intimidate her patients, subordinates and superiors.
- The "Black Boys" Washington, Williams and Warren: Three black men who work as aides in the ward. Williams is a dwarf, his growth stunted after witnessing his mother being raped by white men. The Chief says Nurse Ratched hired them for their sadistic nature.
- Dr. Spivey: The ward doctor. Nurse Ratched drove off other doctors, but she kept Spivey because he always did as he was told. Harding suggests that the nurse could threaten to expose him as a drug addict if he stood up to her. McMurphy's rebellion inspires him to stand up to Nurse Ratched.
- Nurse Pilbow: The young night nurse. Her face, neck and chest are stained with a profound birthmark. She is a devout Catholic and has fear of sinning. She blames the patients for infecting her with their evil and takes it out on them.
- Mr. Turkle: An elderly African American aide who works the late shift in the ward. He agrees to allow McMurphy to host a party and sneak in prostitutes one night.
The acutes are patients who officials believe can still be cured. With few exceptions, they are there voluntarily.
- Billy Bibbit: A nervous, shy and boyish patient with an extreme speech impediment, Billy cuts himself and has attempted suicide numerous times. To alleviate Billy's fear of women, McMurphy sneaks a prostitute into the ward so Billy can lose his virginity. The next morning, Nurse Ratched threatens to tell his mother; fearing the loss of his mother's love, Billy has an emotional breakdown and commits suicide by cutting his own throat.
- Dale Harding: The unofficial leader of the patients before McMurphy arrives, he is an intelligent, good-looking man who's ashamed of his repressed homosexuality. Harding's beautiful yet malcontent wife is a source of shame for him.
- George Sorensen: A man with germaphobia, he spends his days repeatedly washing his hands in the ward's drinking fountain. McMurphy manages to persuade him to lead a fishing expedition for the patients after discovering that he had captained a PT boat during World War II. Afterward, the staff forcibly delouse him, knowing the mental anguish this causes him.
- Charles Cheswick: A loud-mouthed patient who always demands changes in the ward, but never has the courage to see anything through. He finds a friend in McMurphy, who's able to voice his opinions for him. After McMurphy loses his confidence when he learns that his stay in the ward is indefinite, Cheswick drowns himself in the swimming pool.
- Martini: A patient who suffers from severe hallucinations.
- Scanlon: A patient obsessed with explosives and destruction. He is the only other non-vegetative patient confined to the ward by force aside from McMurphy and Bromden; the rest can leave at any time.
- Sefelt and Fredrickson: Two epileptic patients. Jim Sefelt refuses to take his anti-seizure medication, as it makes his teeth fall out. Bruce Fredrickson takes Sefelt's medication and his own because he is terrified of the seizures, and loses teeth due to the resulting overdosage.
- Max Taber: An unruly patient who was released before McMurphy arrived. The Chief later describes how, after questioning what was in his medication, Nurse Ratched had him "fixed."
The Chronics are patients who will never be cured. Many of the chronics are in vegetative states.
- Ruckly: A hell-raising patient who challenges the rules until his lobotomy. After the lobotomy, he sits and stares at a picture of his wife, and occasionally screams profanities.
- Ellis: Ellis was put in a vegetative state by electroshock therapy. He stands against the wall in a disturbing messianic position with arms outstretched.
- Pete Bancini: Bancini suffered brain damage at birth but managed to hold down simple jobs, such as a switch operator on a lightly-used railroad branch line, until the switches were automated and he lost his job, after which he was institutionalized. The Chief remembers how once, and only once, he lashed out violently against the aides, telling the other patients that he was a living miscarriage, born dead.
- Rawler: A patient on the Disturbed ward, above the main ward, who says nothing but "loo, loo, loo!" all day and tries to run up the walls. One night, Rawler castrates himself while sitting on the toilet and bleeds to death before anyone realizes what he has done.
- Old Blastic: An old patient who is in a vegetative state. The first night McMurphy is in the ward, Bromden dreams Blastic is hung by his heel and sliced open, spilling his rusty visceral matter. The next morning it is revealed that Blastic died during the night.
- The Lifeguard: An ex-professional football player, he still has the cleat marks on his forehead from the injury that scrambled his brains. He explains to McMurphy that, unlike prison, patients are kept in the hospital as long as the staff desires.
- Colonel Matterson: The oldest patient in the ward, he suffers from severe senile dementia and cannot move without a wheelchair. He is a veteran of the First World War, and spends his days "explaining" objects through metaphor.
- Candy: The prostitute that McMurphy brings on the fishing trip. Billy Bibbit has a crush on her and McMurphy convinces Candy to sleep with him.
- Sandy: Another prostitute and friend of McMurphy. She and Sefelt sleep together. Sefelt has a seizure while they are having sex.
- Vera Harding: Dale Harding's wife.