Tremadog Bay (also spelled Tremadoc Bay; Welsh: Bae Tremadog), is a shallow inlet of Cardigan Bay in North Wales.^[1] It comprises the area between Mochras (Shell Island) in the east and the Trwyn Cilan headland of the Llŷn Peninsula in the west. The Sarn Badrig reef extends south-west from Mochras and forms a natural barrier between Tremadog Bay and Cardigan Bay proper.^[2]

The Afon Glaslyn flows into the north of the bay and Saint Tudwal's Islands lie at the western end of the bay.^[2] The towns of Pwllheli, Criccieth, Porthmadog, and Harlech lie directly on the bay.