"Old Hundredth" (also known as "Old Hundred") is a hymn tune in long metre, from the second edition of the Genevan Psalter. It is one of the best known melodies in many occidental Christian musical traditions. The tune is usually attributed to the French composer Louis Bourgeois (c. 1510 – c. 1560).

Although the tune was first associated with Psalm 134 in the Genevan Psalter, the melody receives its current name from an association with the 100th Psalm, in a translation by William Kethe titled "All People that on Earth do Dwell". The melody is also sung to various other lyrics, including the Common Doxology ("Praise God From Whom All Blessings Flow") and various German Lutheran chorales. In that latter respect it was used by Johann Sebastian Bach as a cantus firmus in his chorale cantata Herr Gott, dich loben alle wir (BWV 130).

The Genevan Psalter was compiled over a number of years in the Swiss city of Geneva, a center of Protestant activity during the Reformation, in response to the teaching of John Calvin that communal singing of psalms in the vernacular language is a foundational aspect of church life. This contrasted with the prevailing Catholic practice at the time in which sacred texts were chanted in Latin by the clergy only. Calvinist musicians including Bourgeois supplied many new melodies and adapted others from sources both sacred and secular. The final version of this psalter was completed in 1562. Calvin intended the melodies to be sung in plainsong during church services, but harmonized versions were provided for singing at home.

The original lyrics set to this tune in the Genevan Psalter are a paraphrase of Psalm 134:

Or, in English translation:

You faithful servants of the Lord,
sing out his praise with one accord,
while serving him with all your might
and keeping vigil through the night.

Unto his house lift up your hand
and to the Lord your praises send.
May God who made the earth and sky
bestow his blessings from on high.

Old 100th is commonly used to sing the lyrics that begin "All People That on Earth Do Dwell," Psalm 100, a version that originated in the Anglo-Genevan Psalter (1561) and is attributed to the Scottish clergyman William Kethe. Kethe was in exile at Geneva at this time, as the Scottish Reformation was only just beginning. The first verse is as follows:

All people that on earth do dwell,
Sing to the Lord with cheerful voice;
Him serve with mirth, His praise forth tell;
Come ye before Him and rejoice.