C大调是一个于C音开始的音乐的大调，组成的音有C、D、E、F、G、A、B及C。C大调是一个没有升号和降号的调。

它的关系小调是A小调，同名小调是C小调。C大调是音乐中最普通的调号，移调乐器在演奏本身乐器的调性都是以C大调作为它们的记谱。例如，一支降B大调单簧管演奏的乐谱是降B大调，记谱则是C大调。钢琴上的白键都是C大调的音。很多在古典时代写的乐曲都是以C大调为主调，海顿104首交响曲之中有19首就是C大调，这亦是他最常用的调。舒伯特有两首交响乐属于C大调，第一首是"Little C major"、第二首是"the great C major"。

The C major scale consists of the pitches C, D, E, F, G, A, and B. Its key signature has no flats and no sharps.

Its relative minor is A minor and its parallel minor is C minor.

C major is one of the most common key signatures used in western music. Most transposing instruments playing in their home key are notated in C major; for example, a clarinet in B♭ 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. Among brass instruments, the contra-bass tuba is in C. A pedal harp tuned to C major has all of its pedals in the middle position.