Magdalena Sibylla of Neidschütz (8 February 1675 – 14 April 1694), later Countess of Rochlitz, was a German noblewoman and the mistress of John George IV, Elector of Saxony.

Magdalena Sibylla, called Billa, was the daughter of Ursula Margarethe of Haugwitz (1650-1713), who had at one time been the mistress of John George III, Elector of Saxony.

By order of the Elector, Ursula married Colonel Rudolf of Neidschütz (1627-1703), member of an old Saxon nobility, who officially appears as the father of Billa, though there were rumors that Billa was in fact the child of Johann George.

If this had been true, it would have made Billa half-sister of her lover, John George IV.

John George abandoned his wife in the official residence and moved into another palace with Billa.