C major (often just C or key of C) is a musical major scale based on C, with pitches C, D, E, F, G, A, and B. Its key signature has no flats/sharps (see below: Diatonic Scales and Keys).

Its relative minor is A minor, and its parallel minor is C minor. Many instruments, such as the piano, are tuned in C.

C major is one of the most commonly used key signatures in music. Most transposing instruments playing in their home key are notated in C major; for example, a clarinet in B-flat sounding a B-flat major scale is notated as playing a C major scale. The white keys of the piano correspond to the C major scale (however, some electronic keyboards are in B-flat). A harp tuned to C major has all its pedals in the middle position.

C major is often thought of as the simplest key, owing to its lack of either sharps or flats, and beginning piano students' very first pieces are usually very simple ones in this key, and the first scales and arpeggios that students learn are usually C-major ones. However, going against this common practice, the composer Frédéric Chopin regarded this scale as the most difficult one to play with complete evenness, and he tended to give it last to his students. He regarded B major as the easiest scale to play on the piano, because the position of the black and white notes best fitted the natural positions of the fingers, and so he often had students start with this scale. The difficulty of playing a C-major scale evenly would result from the lack of black notes, thus not fitting the natural positions of the fingers so well.

19 of Joseph Haydn's 104 symphonies are in C major, making it his second most often used main key, second only to D major. Of the 134 symphonies mistakenly attributed to Haydn that H. C. Robbins Landon lists in his catalog, 33 of them are in C major, more than any other key. Before the invention of the valve trumpet, Haydn did not write trumpet and timpani parts in his symphonies, except those in C major. H. C. Robbins Landon writes that it wasn't "until 1774 that Haydn uses trumpets and timpani in a key other than C major ... and then only sparingly."

Many Masses and settings of Te Deum in the Classical era were in C major. Mozart wrote most of his Masses in C major.

Of Franz Schubert's two symphonies in the key, the first is nicknamed the "Little C major" and the second the "Great C major."

Although not that difficult for a guitar, C major is not considered ideal for the instrument. The three notes of the dominant chord (G, B & D) are available as open strings, but the root of the tonic chord is not.

French composers such as Marc-Antoine Charpentier and Rameau generally thought of C major as a key for happy music, but Hector Berlioz in 1856 described it as "serious but deaf and dull." Ralph Vaughan Williams was impressed by Sibelius's Symphony No. 7 in C major and remarked that only Sibelius could make the key sound fresh. However, C major was a key of great importance in Sibelius's previous symphonies. Claude Debussy, noted for composing music that avoided a particular key center, once said, "I do not believe in the supremacy of the C major scale."

In musical catalogs that sort the musical pieces by key, whether they go by semitones or along the circle of fifths, they almost always begin with those pieces in C major.

A notable modern use of the key is Terry Riley's In C.

Most slot machines sounds are in C major to avoid sounding dark or sinister.

The default new document in most music notation software (like Finale and Sibelius) is in C major.