Hitchcock insisted that Bernard Herrmann write the score for Psycho despite the composer's refusal to accept a reduced fee for the film's lower budget. The resulting score, according to Christopher Palmer in The Composer in Hollywood (1990) is "perhaps Herrmann's most spectacular Hitchcock achievement."

Hitchcock was pleased with the tension and drama the score added to the film, later remarking "33% of the effect of Psycho was due to the music." The singular contribution of Herrmann's score may be inferred from the unusual penultimate placement of the composer's name in the film's opening credit sequence, as it is followed only by Hitchcock's directing credit. Herrmann used the lowered music budget to his advantage by writing for a string orchestra rather than a full symphonic ensemble, contrary to Hitchcock's request for a jazz score. He thought of the single tone color of the all-string soundtrack as a way of reflecting the black-and-white cinematography of the film. The strings play con sordini (with a muting device placed across the bridge) for all the music other than the shower scene, creating a darker and more intense effect.

Hollywood composer Fred Steiner, in an analysis of the score to Psycho, points out that string instruments gave Herrmann access to a wider range in tone, dynamics, and instrumental special effects than any other single instrumental group would have. The main title music, a tense, hurtling piece, sets the tone of impending violence, and returns three times on the soundtrack.

Though nothing shocking occurs during the first 15-20 minutes of the film, the title music remains in the audience's mind, lending tension to these early scenes. Herrmann also maintains tension through the slower moments in the film through the use of ostinato.