Ynyslas (Welsh for 'Green Island'; Welsh pronunciation: [ənɪsˈlas]) is a small village about 1.5 miles north of Borth and 8 miles north of Aberystwyth, within the county of Ceredigion, Wales. It is sandwiched between a long sandy beach in Cardigan Bay and the beach in the Dyfi Estuary. The area between the sea and the estuary beach is made up of the Ynyslas Sand Dunes which are part of the Dyfi National Nature Reserve and home to many rare plants and animals. The sands of the estuary beach can be driven onto and parked upon. The nature reserve has a visitor centre with toilets and a small shop. At the start of some BBC1 programmes, it shows people flying kites on the sand dunes, this was filmed at Ynyslas.

The northern end of the Ceredigion Coast Path extends to the Dyfi Estuary National Nature Reserve at Ynyslas.

The Dyfi National Nature Reserve and the Ynyslas Sand Dunes are situated where the mouth of the Afon Leri joins the Dyfi Estuary at Ynyslas. Cors Fochno is situated to the east beyond the Afon Leri. At low tide the remains of an ancient submerged forest with stumps of petrified oak, pine, birch, willow and hazel are exposed on the beach.

From 1867 Ynyslas had a railway station on the Cambrian Railways, with sidings serving the riverside wharves. The Plynlimon and Hafan Tramway proposed to create a narrow-gauge line to the wharves in the 1890s but this was never built. Ynyslas railway station was closed by the London Midland Region of British Railways on 14 June 1965.

Borth Rowing Club stores its boat at, and launches from the Ynyslas Boatyard on the banks of the Afon Leri.

The 18 hole golf course at the Borth & Ynyslas Golf Club stretches from Borth to Ynyslas.

Swimming is not allowed at Ynyslas in the sea or estuary, due to the dangerously strong currents.

Ynyslas was the home of the Aberystwyth Beach Cricket Society, under the name the Ynyslas Oval.

• Ynyslas Sand Dunes