The Requiem in D minor, K. 626, is a Requiem Mass by Wolfgang Amadeus Mozart (1756–1791). Mozart composed part of the Requiem in Vienna in late 1791, but it was unfinished at his death on 5 December the same year. A completed version dated 1792 by Franz Xaver Süssmayr was delivered to Count Franz von Walsegg, who had commissioned the piece for a requiem service on 14 February 1792 to commemorate the first anniversary of the death of his wife Anna at the age of 20 on 14 February 1791.

The Köchel catalogue is a chronological catalogue of compositions by Wolfgang Amadeus Mozart, originally created by Ludwig Ritter von Köchel, in which the entries are abbreviated K. or KV. Its numbers reflect the ongoing task of compiling the chronology of Mozart's works, and provide a shorthand reference to the compositions.