A type of button accordion on which the melody-side keyboard contains one or more rows of buttons, with each row producing the notes of a single diatonic scale. The buttons on the bass-side keyboard are most commonly arranged in pairs, with one button of a pair sounding the fundamental of a chord and the other the corresponding major triad (or, sometimes, a minor triad). 

Diatonic button accordions are popular in many countries and used mainly for playing popular music and traditional folk music, and modern offshoots of these genres.

In some countries the name melodeon is often used for a diatonic accordion, although this is strictly a particular type.

Most diatonic button accordions have a "single-action" (or bisonoric) keyboard, meaning that each button produces 2 notes: one when the bellows are pressed or pushed (closed) and another when the bellows are drawn or pulled (opened). This is similar to the concertina and the harmonica. Because each button produces 2 notes, the diatonic scale can be covered in 4 buttons on a melody row.