The Missa solemnis in C minor, K. 139/47a, is a mass composed by Wolfgang Amadeus Mozart in the summer of 1768 in Vienna.

The mass was commissioned by the Jesuit priest Father Ignaz Parhammer, who asked Mozart for music for the consecration of the new Orphanage Church – Waisenhauskirche [de] – on the Rennweg.

Mozart also composed a trumpet concerto suitable for performance by a boy as well as an offertory, both thought to be lost.

The twelve-year-old Mozart conducted a choir of orphans in a performance that received "universal acclaim and admiration".

[2] This mass is considered Mozart's most ambitious work to be performed up until that point in time,[5] and was his first and longest missa longa.