Glynn ap Derfel (born 844) was a Welsh brigand during the late 9th century. He was a farmer before leading a band of fellow serfs in escaping the farm and forming the "free brothers", a band of brigands who preyed on travellers around Tref Went. In 868, he and his band were recruited into Bjorn Tall-Trunks' warband for 1,152 peningas.