In music theory, a leading tone (also called subsemitone or leading note in the UK) is a note or pitch which resolves or "leads" to a note one semitone higher or lower, being a lower and upper leading tone, respectively. Typically, the leading tone refers to the seventh scale degree of a major scale (), a major seventh above the tonic. In the movable do solfège system, the leading tone is sung as si.

A leading-tone triad is a triad built on the seventh scale degree in a major key (vii^o in Roman numeral analysis), while a leading-tone seventh chord is a seventh chord built on the seventh scale degree (vii^ø7). Walter Piston considers and notates vii^o as V⁰
₇, an incomplete dominant seventh chord. (For the Roman numeral notation of these chords, see Roman numeral analysis.)

Typically, when people speak of the leading tone, they mean the seventh scale degree () of the major scale, which has a strong affinity for and leads melodically to the tonic. It is sung as si in movable-do solfège. For example, in the F major scale, the leading note is the note E.

As a diatonic function, the leading tone is the seventh scale degree of any diatonic scale when the distance between it and the tonic is a single semitone. In diatonic scales in which there is a whole tone between the seventh scale degree and the tonic, such as the Mixolydian mode, the seventh degree is called the subtonic. However, in modes without a leading tone, such as Dorian and Mixolydian, a raised seventh is often featured during cadences, such as in the harmonic minor scale.

A leading tone outside of the current scale is called a secondary leading tone, leading to a secondary tonic. It functions to briefly tonicize a scale tone (usually the 5th degree) as part of a secondary dominant chord. In the second measure of Beethoven's Waldstein Sonata (shown below), the F♯'s function as secondary leading tones, which resolve to G in the next measure.

By contrast, a descending, or upper, leading tone is a leading tone that resolves down, as opposed to the seventh scale degree (a lower leading tone) which resolves up. The descending, or upper, leading tone usually is a lowered second degree (♭) resolving to the tonic, but the expression may at times refer to a ♭ resolving to the dominant.^{[citation needed]} In German, the term Gegenleitton ("counter leading tone") is used by Hugo Riemann to denote the descending or upper leading-tone (♭), but Heinrich Schenker uses abwärtssteigenden Leitton ("descending leading tone") to mean the descending diatonic supertonic (♮).)

The tritone substitution, chord progression ii–subV–I on C (Dm–Db7–C), results in an upper leading note.

According to Ernst Kurth, the major and minor thirds contain "latent" tendencies towards the perfect fourth and whole tone, respectively, and thus establish tonality. However, Carl Dahlhaus contests Kurth's position, holding that this drive is in fact created through or with harmonic function, a root progression in another voice by a whole tone or fifth, or melodically (monophonically) by the context of the scale. For example, the leading tone of alternating C chord and F minor chords is either the note E leading to F (if F is tonic), or A♭ leading to G (if C is tonic).