German composer. Entered the Leipzig Thomasschule in 1685 for a seven-year program, studying under Thomaskantor Schelle and possibly with Thomaskirche organist Kuhnau; the dates of his departure from Leipzig and his engagement at the Brunswick court are unknown. His first stage work, Der königliche Schäfer was performed at Braunschweig between 1692 and 1694; he was appointed chamber composer there in 1694. He soon moved to Hamburg, supplying several operas (mostly to librettos by Postel) to the Oper am Gansemarkt during G. Schott's time as director (1695-1702). Schott's widow passed the opera directorship to Keiser in 1702; he teamed with the writer Drüsicke to manage the theater until 1707, during which time many operas of his own were given along with those of Handel, Mattheson, and Gottfried Grünewald. After relinquishing the theater (which had gone into debt) in 1707, he served several Holstein nobles during the next year; his opera Der Karneval von Venedig (1707), with substitute arias by Graupner and other songs in local Hamburg dialect met with great success, playing for over a year. He wrote at least 25 operas for the Oper am Gansemarkt between 1708 and 1718; his 1712 setting of Brockes's Passion-oratorio text was the first of a string of settings by several composers, including Handel, and was central to the development of the genre. All of Keiser's oratorios date from roughly this period, as do several collaborative efforts with Mattheson.
He was in Stuttgart in 1719-20 but failed to secure a permanent court position. He returned to Hamburg in August 1721, but Telemann's newly established dominance there caused him to seek a post at Copenhagen; this too ultimately proved fruitless, though he staged three of his operas there in 1722. He spent his last years back in Hamburg, writing several operas as well as church music; in 1728 was named to succeed Mattheson as Canonicus minor and cantor at the cathedral. Mattheson and Telemann paid him high tribute in obituaries upon his death, while Scheibe and Hasse acknowledged his great talent and influence many years later (the latter comparing him favorably with Domenico Scarlatti); Burney wrote that his works showed "all the vigor of a fertile invention, and correctness of study and experience." He numbered his operas at over 100, but this probably includes pasticcios and smaller works.