Cemlyn Bay is a bay on the northwest coast of Anglesey, west of Wylfa nuclear power station. The beach is pebbly and has sand at low tide, but most people don’t visit for the beach; they come for what lies behind it. Separating the beach is a brackish lagoon fed by several small streams. A weir at the western (Bryn Aber) end of the beach regulates the lagoon’s water level.

The site is designated a Site of Special Scientific Interest and is currently part of the Anglesey Heritage Coast and the Isle of Anglesey Area of Outstanding Natural Beauty. The Cemlyn estate is owned by the National Trust; the lagoon and its immediate surrounds comprise Cemlyn Nature Reserve (25.2 ha in extent, set up in 1971 and leased by the North Wales Wildlife Trust).