Agnes of Ponthieu (c. 1080 – aft. 1105) was the daughter of Count Guy I of Ponthieu. Enguerrand, the son of Count Guy, died at a youthful age. Guy then made his brother Hugh heir presumptive, but he also died before Guy (died 1100). Agnes became count Guy's heiress, and was married to Robert of Bellême. Their son William III of Ponthieu succeeded to the county of Ponthieu after the death of Agnes (between 1105 and 1111), and the imprisonment of his father in 1112.