The Fugue in G minor, BWV 131a, is a piece of organ music attributed to Johann Sebastian Bach. It is a transcription of the last movement of the cantata Aus der Tiefen rufe ich, Herr, zu dir, BWV 131, which features a permutation fugue. The cantata dates from 1707 or 1708, which almost certainly provides a terminus ante quem for the organ arrangement.
G minor is used extensively in the cantata, which sets one of the penitential psalms. In the cantata the fugue is sung by the choir. The score of the cantata does not feature an organ part as such, but the basso continuo (for which a figured bass is provided) may well have been played on the organ. The cantata is definitely by Bach, while the arrangement for organ is regarded by some authorities (from Spitta onwards) as spurious.