Low Mass (Latin Missa Privata) is a Mass celebrated by a priest without the assistance of sacred ministers (deacon and subdeacon). Before the 1969 reforms, a sub-distinction was also made between the sung Mass (Missa in cantu), when the celebrant still chants those parts which the rubrics require to be chanted, and the low Mass (Missa lecta) where the liturgy is spoken.

In a low Mass, the priest may be assisted by altar boys (acolytes) rather than deacons, and use appropriately simplified rubrics.

A full sung Mass celebrated with the assistance of sacred ministers is a High or Solemn Mass.

The celebration of Low Mass in the Roman Rite ended with the 1969 reforms in the Catholic Church, but continues in Lutheranism and parts of Anglicanism.

Low Mass originated in the early Middle Ages as a shortened or simplified form of Solemn Mass. In the early church, as in the Eastern Orthodox church today, all services were chanted, and there was no equivalent to the Roman Low Mass or to the Anglican "said celebration".

Alongside the public solemn Masses, the practice developed from the 4th century onwards, of smaller private Masses for smaller groups of believers. These masses were often celebrated in the catacombs, for the deceased or on a special anniversary. An example is provided by Saint Augustine:

Hesperius, of a tribunitian family, ... finding that his family, his cattle, and his servants were suffering from the malice of evil spirits, he asked our presbyters, during my absence, that one of them would go with him and banish the spirits by his prayers. One went, offered there the sacrifice of the body of Christ, praying with all his might that that vexation might cease. It did cease immediately, through God's mercy.

— Saint Augustine of Hippo, The City of God, Book 22, chapter 8, n. 6.