Bernhard Christian Kayser was a German organist, chamber musician and lawyer. He received private instruction in organ from J.S. Bach in Köthen in 1717-1720. When J.S. Bach moved to Leipzig, in 1723, Kayser followed him, continued his instruction with J.S. Bach, undertook the study of law at the University of Leipzig in 1724, and for a number of years was apparantly one of J.S. Bach's closet associates, perhaps even serving for a time as J.S. Bach's private secretary. Returning to Köthen in 1730, he functioned at the court of the Prince from 1733 at the latest as "lawyer to the court government" as well as "chamber musician" and court organist, for a time probably as organist of St. Agnus's Church, of which he was a member.
During his seven years in Leipzig (1723-1730), Bernhard Christian Kayser copied various organ and piano works and cantatas of J.S. Bach. He was a very able student, who made many more copies of J.S. Bach's organ and keyboard works than Anna Magdalena Bach; his handwriting was often indistinguishable from J.S. Bach’s. Important copies of J.S. Bach's stem from Kayser, including, for example, the Prelude & Fugue in E minor, BWV 548, which attests to Kayser's musical capabilities.