In music, sharp – eqv. dièse (from French) or diesis (from Greek δίεσις) – means higher in pitch. The sharp symbol, ♯, indicates that the note to which the symbol is applied is played one semitone higher. The opposite of sharp is flat, indicating a lowering of pitch. The symbol derives from a square form of the letter b.

The sharp symbol is used in key signatures or as an accidental applied to a single note. Below is a staff with a key signature containing three sharps (A major or F♯ minor) and a sharp symbol placed on the note, indicating that it is an A♯ instead of an A♮.

In twelve-tone equal temperament tuning (the predominant system of tuning in Western music), raising a note's pitch by a semitone results in a note that is enharmonically equivalent to another named note. For example, E♯ and F would be equivalent. This is not the case in most non-standard tuning systems.

A double sharp is indicated by the symbol and raises a note by two semitones (a whole tone). Double sharps were sometimes written , or .

A half sharp or demisharp () raises a note by approximately a quarter tone. A sharp-and-a-half, three-quarter-tone sharp or sesquisharp () raises a note by three quarter tones.

A triple sharp ( or ♯) is extremely rare. It would raise a note by three semitones (a whole tone plus a semitone). The B below would be enharmonic with D natural.

While this system allows for higher multiples of sharps, triple sharps are the practical limit, and there are only a few examples in the literature. In other tuning systems, such as 53 equal temperament, quadruple sharps or beyond may be required. A quadruple sharp would be indicated by the symbol .^{[citation needed]}

In a key signature, sharps or flats are placed to the right of the clef. The pitches indicated apply in every measure and octave.