Boulogne-sur-Mer became the centre of the County of Boulogne in the 9th century. The founder of the dynasty of the counts of Boulogne seems to have been Hernequin of Boulogne, the son of Ragnhart; Hernequin married Bertha of Ponthieu around 850. Later that century it was frequently raided by the Vikings. There is some uncertainly about the early counts; there are number of people called "count" but the first definite count does not appear until the 11th century.

Boulogne later became influential in the history of England, when Eustace II accompanied William the Conqueror's invasion in 1066. Boulogne was also a major participant in the First Crusade; Eustace III's brothers Godfrey and Baldwin both became king of Jerusalem, and Eustace himself was offered but declined the title.

Count Renaud joined the imperial side at the Battle of Bouvines in 1214, and was defeated by Philip II of France. Boulogne passed under nominal royal control in 1223 when it was given to Philip II's son Philip Hurepel. Hurepel revolted against Blanche of Castile when Louis VIII died in 1226. When Philip died in 1235, for unknown reasons the county passed to Adelaide of Brabant, Matilda's niece, and her husband William X, count of Auvergne, rather than Matilda and Philip's descendants.

Boulogne was attacked numerous times during the Hundred Years' War. In 1477 Bertrand VI of La Tour gave up the county to Louis XI who incorporated it into France (except for a brief period of English rule under Henry VIII; see Siege of Boulogne).

Counts of Boulogne

Uncertain counts

• Hernequin
• Odakar V "Le Grand" (?-?)
• Inglebert I (?-?)
• Odakar VI (?-?)
• Arnoul II (?-?)
• Baldwin I (896-918) (also Count of Flanders)
• Adalolf (918-933)
• Guy (933-987)
• Hugh III (987-?)
• Baldwin II (?-1032)

Certain counts

• Eustace I (1032-1049)
• Eustace II (1049-1093)
• Eustace III (1093-1125)
• Matilda I (1125-1151)
  • Stephen (1128-1150)
• Eustace IV (1151-1153)
• William of Blois (1153-1159) (also Earl of Surrey)
• Marie (1159-1170)
  • Matthew I (1160-1173)
• Ida (1173-1216)
  • Matthew II (1173-1180)
  • Gerard (1181-1182)
  • Berthold (1183-1186)
  • Renaud (1190-1216)
• Matilda II (1216- c. 1260)
  • Philip Hurepel (also Count of Clermont ) (1223-1235)
  • Alphonse (1235-1253)
• Adelaide of Brabant (c. 1260-1261) (daughter of Henry I, Duke of Brabant)
• Robert V of Auvergne (1261-1277)
• Robert VI of Auvergne (1277-1314)
• Robert VII of Auvergne (1314-1325)
• William XII of Auvergne (1325-1332)
• Joanna I of Auvergne (1332-1360)
  • Philip of Burgundy (1338-1346)
  • John II of France (1350-1360)
• Philip (1360-1361) (also Duke of Burgundy)
• John I of Auvergne (1361-1386)
• John II of Auvergne (1386-1404)
• Joanna II of Auvergne (1404-1424)
  • John I of Berry (1404-1416)
  • George of La Tremoille (1416-1424)
• Marie I of Auvergne (1424-1437)
• Bertrand V of La Tour (1437-1461)
• Bertrand VI of La Tour (1461-1497)