Plas Newydd (Welsh for 'new hall') is a country house set in gardens, parkland and surrounding woodland on the north bank of the Menai Strait, in Llanddaniel Fab, near Llanfairpwllgwyngyll, Anglesey, Wales.

Owned successively by Griffiths, Baylys and Pagets, it became the country seat of the Marquesses of Anglesey, and the core of a large agricultural estate.

Henry Paget, 7th Marquess of Anglesey presented it to the National Trust in 1976 so that the house and grounds could be opened to the public.

[2] In 1533 Ellen Griffith married Nicholas Bagenal and they took possession of what was still known as Llwyn-y-Moel, and the property was sold or mortgaged to Henry Bagnall about 1575.

Lewis's great-grandson Sir Nicholas Bayly, 2nd Baronet married Caroline Paget in 1737 and became Lord Lieutenant of Anglesey.

Plas Newydd had been extended in the middle of the 18th century, with an octagonal tower at the south-east corner, but under Henry's care there were substantial additions and rebuilding throughout the estate, especially with the appointment of James Wyatt and Joseph Potter as architects.

Henry William had raised a regiment of volunteers in the 1790s, was commissioned into the Army in 1795, and distinguished himself in numerous engagements and campaigns across Europe.

Renowned for his lavish spending and flamboyant lifestyle, he converted the family chapel inside the house into a performance space called the Gaiety Theatre.

The family also sold off their main home at Beaudesert and their London house, and moved into Plas Newydd permanently.

The ship was wrecked after running aground in 1953, and the school built temporary facilities in the grounds near the current reception centre.

It was excavated by Wilfrid James Hemp in 1929–35, and Neolithic pottery appeared to lie under and in front of the mound, suggesting a settlement predating its use as a burial site.

[23] In addition to the Grade I listed main house, Plas Newydd had a large estate covering 3,848 hectares (9,510 acres) including outlying properties.

Thomas John Wynn, 5th Baron Newborough gave them as a wedding present to Lord and Lady Anglesey in 1948.

Home Farm, Plas Llanedwen and the Church have two entrance gateways and lodge buildings, off the road that forms the estate's south-western boundary.

[48] Its oil-fired boiler made the mansion the most polluting[47] and biggest oil consumer[48] of the National Trust's properties;[47] the renovation is expected to save around £40,000 a year in operating costs.

[47] Plas Newydd is one of five properties in a pilot experiment;[47][48] if they succeed, the National Trust will invest in 43 more renewable energy plans.

[47][48] The pilot programme includes: biomass in Croft Castle in Herefordshire, a woodchip boiler in Ickworth in Suffolk, and hydroelectric projects in Hafod y Porth near Craflwyn in Snowdonia,[50] and at Stickle Ghyll in the Lake District.