Sibylle of Cleves (1512 — 1554) was a sister-in-law of Henry VIII who is all but forgotten. She was born on 17 January 1512 in Düsseldorf and was the daughter of John III, Duke of Cleves and Count of Mark, and Maria of Jülich-Berg, daughter of the Duke of Jülich-Berg. Sibylle was named after her maternal grandmother, Sibylle of Brandenburg.

Sibylle was her parent’s first child, born a few years after their marriage in 1509. She was followed by a younger sister, Anne of Cleves, in 1515, who briefly became Queen of England. The next year, in 1516, Sibylle’s only brother, William, the future Duke of Jülich-Cleves-Berg and husband of one of Emperor Ferdinand I’s daughters, was born. Through William, Sibylle would become a distant relative to two Queens of Sweden: Maria Eleonora of Brandenburg, William’s great-granddaughter, and her daughter, Christina of Sweden. The final sibling, another sister, Amalia, remained unmarried, though she was considered as a bride for Henry VIII, he chose Anna instead.

In 1526, Sibylle was betrothed to John Frederick of Saxony, son of the Elector of Saxony, one of the men who would elect the Holy Roman Emperor. They married in a very extravagant ceremony on 9 February 1527, when Sibylle was fifteen years old. John Frederick became Elector of Saxony and Sibylle became Electress following her father-in-law’s death.

Sibylle and John Frederick most likely had a happy enough marriage and had four sons: John Frederick, John William, John Ernst (who died a few days after his birth), and another John Frederick. During the Schmalkaldic War, the city of Wittenberg was besieged. Sibylle and John Frederick both made efforts to save Wittenberg, and eventually, as a cause of the siege, John Frederick was forced to abdicate in 1547, giving the throne to his relative, Maurice of Saxony.

After his abdication, John Frederick was imprisoned. Five years later, in 1552, he was released and could return to his family. After enjoying a short period of happiness, in 1554, Sibylle died on 21 February in Weimar. John Frederick followed her to the grave on 3 March 1554. Sibylle was forty-two, and her husband was fifty. Their sons, both named John Frederick, became Dukes of Saxony, while their other surviving son, John William, became Duke of Saxe-Weimar. Sibylle and her husband are buried in the City Church of Weimar.