The washtub bass, or gutbucket, is a stringed instrument used in American folk music that uses a metal washtub as a resonator. Although it is possible for a washtub bass to have four or more strings and tuning pegs, traditional washtub basses have a single string whose pitch is adjusted by pushing or pulling on a staff or stick to change the tension.

The washtub bass was used in jug bands that were popular in some African American communities in the early 1900s. In the 1950s, British skiffle bands used a variant called a tea chest bass, and during the 1960s, US folk musicians used the washtub bass in jug band-influenced music.

Variations on the basic design are found around the world, particularly in the choice of resonator. As a result, there are many different names for the instrument including the "gas-tank bass", "barrel bass", "box bass" (Trinidad), "bush bass" (Australia), "babatoni" (South Africa), "tanbou marengwen" (Haiti) "tingotalango" (Cuba), "tulòn" (Italy), "laundrophone" and others.

The hallmarks of the traditional design are simplicity, very low cost and do it yourself construction, leading to its historical association with lower economic classes. These factors also make it quite common for modern-day builders to promote modifications to the basic design, such as adding a finger board, pedal, electronic pickup, drumhead, or making the staff immovable.