Gui de Maillesec (d. March 8, 1412 at Paris) was a French bishop and cardinal. He played a major part in the election of the antipope Benedict XIII, in his status as senior cardinal. His name is also written Malésec or Malesset.
He was Bishop of Poitiers. In 1375 he was appointed to the Church of the Holy Cross in Jerusalem, also being made a cardinal by Gregory XI. He was then bishop of Palestrina from 1384 to 1412. Given the schism of the time, his appointment from Avignon was contested, by Francesco Moricotti Prignani, Archbishop of Pisa.
He was dean of the Sacred College of Cardinals in the obedience of Avignon from August 1405 until June 1409, when he took part in the Council of Pisa and the election of "Pisan" Antipope Alexander V. He retained the deanship of the College in the obedience of Pisa.