William IX (Occitan Guilhem or Guillem) was the lord of Montpellier from 1202 until 1204. He was the last lord of the Guilhem lineage. William IX was the eldest son of William VIII of Montpellier. His mother, Agnes (Spanish Inés), was related to the kings of Aragon. She was William VIII's second wife, married in 1187 after he left his first wife, Eudokia Komnene, who was the mother of his daughter and heiress, Mary. William VIII struggled to get the church, the townspeople and neighbouring lords to recognise the legitimacy of his second marriage and his son. William IX had five full brothers and two full sisters.