Although the Welsh royal lineage died out in the 15th century when Owain Glyndwr went into hiding, Bardsey hung on to its own king until 1927.
But now after a 70-year break from the monarchy, the island's trust has decided, after due constitutional consideration, to restore their crowned head of state. Mr Terfel's subjects will be a shepherd, his wife, a nun, and the two other people who live on the island and look after it for the trust.
The children at Crud y Werin school in Aberdaron, on the mainland, have made him a crown and the coronation will take place at the National Eisteddfod on Anglesey next month. Two pupils from the school will perform the ceremony.
Simon Glyn, the director of the island's trust, said Mr Terfel had originally asked to be patron of the trust and when he accepted, its 480 members offered him the crown by way of thanks. "We chose Bryn because he is without doubt Wales's greatest ambassador and his roots are firmly on the peninsula."
Bardsey island has 15 houses and a place called Plas Bach, which means Little Palace. During the winter the population numbers five but in the summer it soars to 50.
Despite the solemn ceremony next month, Mr Terfel will only be the symbolic head of the island and not an all powerful ruler. Nor will he be entitled to the original crown which is at Liverpool's Maritime Museum.
That crown, which is made from brass and tin, was last worn by King Love Pritchard, who died in 1927 and ended the line of Bardsey kings. When he visited the mainland in 1925, he was welcomed by the great Welsh statesman David Lloyd George as an "overseas king". The island declared itself to be a neutral state in the First World War after King Love was refused entry into the British forces for being too old.
The future king was out of the country yesterday and not available to comment. He will return in time for his coronation.