The Missa solemnis in C major, K. 66, is a mass composed by Wolfgang Amadeus Mozart in 1769. It is scored for SATB soloists, SATB choir, violins I and II, viola, 2 oboes, 2 horns, 2 clarini (high trumpets), 2 trumpets and basso continuo.
Mozart composed the mass for the ordination of Cajetan Hagenauer, son of Lorenz Hagenauer, the Mozarts' landlord and family friend. Cajetan had entered St Peter's Abbey, a Benedictine monastery, whilst the Mozarts were on their grand tour. When he became a priest in 1769, he took the name Pater Dominicus and celebrated his first solemn high mass; this gives the work its nickname, Dominicusmesse or Pater Dominicusmesse. Leopold Mozart conducted the première of the mass to a packed Jesuit Church in Salzburg; it is thought that the success of this performance led to Wolfgang's first Italian tour.
Due to its length, the mass is classified as a missa solemnis. It is divided into six movements.