Sibylla of Anjou (c. 1112–1165) was a countess consort of Flanders as the wife of Thierry, Count of Flanders. She served as the regent of Flanders during the absence of her spouse from 1147 to 1149.

Sybilla was the daughter of Fulk V of Anjou^[1] and Ermengarde of Maine,^[2] In 1123, she married William Clito, son of the Norman Robert Curthose and future Count of Flanders.^[3] Sibylla brought the County of Maine to this marriage, which was annulled, narrowly, in 1124 on grounds of consanguinity.^[3] The annulment was made by Pope Calixtus II^[4] upon request from Henry I of England, William's uncle;^[3] Fulk opposed it and did not consent until Calixtus excommunicated him and placed an interdict over Anjou.^[4]

In 1134, Sibylla married Thierry, Count of Flanders.^[5] During his absence on the Second Crusade the pregnant Sibylla acted as regent of the county.^[6] Baldwin IV, Count of Hainaut took the opportunity to attack Flanders,^[6] but Sibylla led a counter-attack and pillaged Hainaut. In response Baldwin ravaged Artois. The archbishop of Reims intervened and a truce was signed, but Thierry took vengeance on Baldwin when he returned in 1149.

In 1157 Sibylla travelled with Thierry on his third pilgrimage, but after arriving in Jerusalem she separated from her husband and refused to return home with him.^[7] She became a nun at the Convent of Sts. Mary and Martha in Bethany,^[8] where her step-aunt, Ioveta of Bethany, was abbess. Ioveta and Sibylla supported Queen Melisende and held some influence over the church, and supported the election of Amalric of Nesle as Latin Patriarch of Jerusalem over a number of other candidates. Sibylla died in Bethany in 1165.^[9]

Sibylla and Thierry had:

• Philip, Count of Flanders^[10]
• Matthew, Count of Boulogne,^[11] married Marie of Boulogne
• Margaret, Countess of Flanders and Hainaut, married Baldwin V, Count of Hainaut^[11]
• Gertrude, married Humbert III, Count of Savoy^[11]
• Matilda^[11]
• Peter^[11]