The Missa solemnis in D major, Op. 123, is a solemn mass composed by Ludwig van Beethoven from 1819 to 1823. It was first performed on 7 April 1824 in St. Petersburg, Russia, under the auspices of Beethoven's patron Prince Nikolai Galitzin; an incomplete performance was given in Vienna on 7 May 1824, when the Kyrie, Credo, and Agnus Dei were conducted by the composer. It is generally considered one of the composer's supreme achievements and, along with Bach's Mass in B minor, one of the most significant Mass settings of the common practice period.
Despite critical recognition as one of Beethoven's great works from the height of his composing career, Missa solemnis has not achieved the same level of popular attention that many of his symphonies and sonatas have enjoyed. Written around the same time as his Ninth Symphony, it is Beethoven's second setting of the Mass, after his Mass in C major, Op. 86.
The mass is scored for 2 flutes; 2 oboes, 2 clarinets (in A, C, and B♭); 2 bassoons; contrabassoon; 4 horns (in D, E♭, B♭ basso, E, and G); 2 trumpets (D, B♭, and C); alto, tenor, and bass trombone; timpani; organ continuo; strings (violins I and II, violas, cellos, and basses); soprano, alto, tenor, and bass soloists; and mixed choir.