On 15 June 1204 Marie married Peter II of Aragon and thanks to a revolt against her half-brother William IX, she was recognised as Lady of Montpellier.
From her marriage with Peter II, Marie gave birth two children: Infanta Sancha (born in 1205, died aged one) and Infante James, the future King James I the Conqueror (born on 1 February 1208). Peter II immediately attempted to divorce her, hoping both to marry Maria of Montferrat, Queen of Jerusalem, and to claim Montpellier for himself. Marie's last years were spent in combating these political and matrimonial manoeuvres. Pope Innocent III finally decided in her favour, refusing to permit the divorce. Marie died in Rome (21 January 1213) in her way back to Aragon, and Peter II a few months later (14 September 1213) at the Battle of Muret. Marie and Peter II's only surviving child, James I, inherited Aragon and Montpellier.