Slendro (called salendro by the Sundanese) is a pentatonic (five tone) scale, one of the two most common scales used in Indonesian gamelan music. It is more accessible to the western ear than pelog, the other common scale, because it is closer to the pitch relationships often found in western music. Western listeners often think it sounds like the pentatonic scale as played on the black keys of a piano, but out of tune.

The instruments in each gamelan (which means ensemble) are tuned to each other, and the tuning of an individual ensemble may differ from other ensembles of the same variety in the same region. From one region of Indonesia to another the slendro scale often varies widely. The amount of variation also varies from region to region. For example, Slendro in Central Java varies much less from gamelan to gamelan than it does in Bali, where ensembles from the same village may be tuned very differently.

Slendro in Java is close to being five near equally spaced tones to the octave. Therefore it has narrow fourths, in contrast to the wide fourths of the pelog scale. However, analyzing the mathematic relationship of the fundamental frequencies of the keys of the gamelan's metallophones is fairly recent phenomenon and not the traditional way of thinking about slendro.

For experienced participants in gamelan music, the pelog and slendro scales each have a particular feeling, related to the rituals and circumstances in which the scale is used. For example, in Bali, slendro is felt to have a sad sound because it is used as the tuning of gamelan angklung, the traditional ensemble for cremation ceremonies.

The origin of the slendro scale is unknown. It is similar to scales used in Indian and Chinese music as well as other areas of Asia and they all may have a common origin. This is very difficult if not impossible to determine. Even within Indonesia it is difficult to determine the evolution of scales. For example, scales used in Banyuwangi, at the eastern tip of Java, are very similar to scales used in Jembrana, a short distance away on Bali. There is probably no way to document which region influenced the other, or if they both evolved together.