Mochras Lagoon, also referred to as the Artro estuary, is the tidal inlet where the Afon Artro meets the sea on the Cardigan Bay coast of Gwynedd, Wales, situated between Llandanwg and Mochras (Shell Island).

The present estuary took shape after 1819, when the Earl of Winchilsea diverted the river northward to improve access to the slate-shipping wharf at Pensarn; before that engineering work, the Artro entered the sea roughly a mile to the south.

The harbour channel at Mochras dries at low water. The approach runs between the southern tip of Shell Island and pilings to the north, with a red-topped post to port and a series of green buoys to starboard marking the channel.

Passage is considered straightforward in fine weather on a rising tide. The lagoon is recognised as a distinct coastal feature within Cardigan Bay and is noted for its physical and biological character as a tidal estuary environment.