In music theory, a diatonic scale (from the Greek ??????????, meaning "[progressing] through tones", also known as the heptatonia prima and set form 7-35) is a seven-note musical scale comprising five whole-tone and two half-tone steps, in which the half-steps are maximally separated. Thus between each of the two half-steps lie either two or three whole-steps, with the pattern repeating at the octave. The term diatonic originally referred to the diatonic genus, one of the three genera of the ancient Greeks.

These scales are the foundation of the European musical tradition. The modern major and minor scales are diatonic, as were all of the 'church' modes. What are now called major and minor were, during the medieval and Renaissance periods, only two of seven modes formed by a diatonic scale beginning on each of the seven notes of the octave; thus, the half-steps were positioned at different distances from the starting tone in each of these seven scales. By the start of the Baroque period, the notion of musical key was established?based on a central triad rather than a central tone. Major and minor scales came to dominate until at least the start of the 20th century, partly because their intervallic patterns are suited to the reinforcement of a central triad. Some church modes survived into the early 18th century, as well as appearing occasionally in classical and 20th century music, and later in modal jazz.

Using the 12 notes of the chromatic scale, 12 major and 12 minor scales can be formed. The modern musical keyboard, with its black notes grouped in twos and threes, is essentially diatonic; this arrangement not only helps musicians to find their bearings on the keyboard, but simplifies the system of key signatures compared with what would be necessary for a continuous alternation of black and white notes. The black (or "short") keys were an innovation that allows the adjacent positioning of most of the diatonic whole-steps (all in the case of C major), with significant physical and conceptual advantages.