structural:expansion, structural:scholar_edition, quality:correction
The Mass in B minor (German: h-Moll-Messe), BWV 232, is an extended setting of the Mass ordinary by Johann Sebastian Bach. The composition was completed in 1749, the year before the composer's death, and was to a large extent based on earlier work, such as a Sanctus Bach had composed in 1724. Sections that were specifically composed to complete the Mass in the late 1740s include the "Et incarnatus est" part of the Credo.

As usual for its time, the composition is formatted as a Neapolitan mass, consisting of a succession of choral movements with a broad orchestral accompaniment, and sections in which a more limited group of instrumentalists accompanies one or more vocal soloists. Among the more unusual characteristics of the composition is its scale: a total performance time of around two hours, and a scoring consisting of two groups of SATB singers and an orchestra featuring an extended winds section, strings and continuo. Its key, B minor, is rather exceptional for a composition featuring natural trumpets in D, although far more of the work is in this key than B minor.

Even more exceptional, for a Lutheran composer such as Bach, is that the composition is a Missa tota. In Bach's day, Masses composed for Lutheran services usually consisted only of a Kyrie and Gloria. Bach had composed five such Kyrie–Gloria Masses before he completed his Mass in B minor: the Kyrie–Gloria Masses, BWV 233–236, in the late 1730s, and the Mass for the Dresden court, which would become Part I of his only Missa tota, in 1733. The Mass was likely never performed in its entirety during Bach's lifetime. Its earliest documented complete performance took place in 1859. With many dozens of recordings, it is among Bach's most popular vocal works.

In 2015, Bach's personal handwritten manuscript of the mass held by the Berlin State Library was included in the UNESCO 's Memory of the World Register, a project to protect and preserve culturally significant documents and manuscripts.