Llywelyn Siôn (1540 – c. 1616) was a Welsh language poet and bard.
His instructors included Meurig Dafydd and Thomas Llewelyn. Around 1575, he is mentioned under the name Lewelyn John by Sir Edward Mansel in his History of the Norman Conquest of Glamorgan, as a learned and diligent collector of Welsh manuscripts. Following in Meurig Dafydd's footsteps, Siôn became (in 1580) president of the 'Gorsedd' or bardic congress of Glamorgan. He presided at the Glamorgan gorsedd at Tir Iarll in 1580, and was commissioned to collect and publish and traditional lore of the bardic order.