Baroness Sybille de Selys Longchamps (born 28 August 1941) is a Belgian aristocrat.
She is the mother of Delphine Boël, who she claims to be the illegitimate daughter of Albert II of Belgium, former King of the Belgians.
Born in Uccle, Belgium, she is the daughter of Count and ambassador Michel François de Selys Longchamps (1910–1983) and Countess Pauline Cornet de Ways-Ruart (1914–1953).
They divorced in 1978 and in 1982 Sybille married a wealthy British widower, the Honourable Michael Anthony Rathborne Cayzer (1929–1990), a younger son of shipping tycoon Herbert Cayzer, 1st Baron Rotherwick.
In 1968, Delphine Boël was born, allegedly from an extramarital affair with Albert of Belgium, who was not king at the time.
The purported affair was made public in 1999.
Sybille lived with her second husband in London and at his rural estate until the early 1990s and lives now in Brussels and the Provence.