The star of Silence of the Lambs and Nixon, pledged allegiance to the US in a short ceremony earlier this month at the Federal Building in Los Angeles.
His friends Steven Spielberg and John Travolta witnessed the ceremony.
But the news mystified people living in his south Wales home town of Port Talbot and there were even been calls for him to be stripped of the freedom of the town.
But, Sir Anthony explained: "America has been very generous to me, magnanimous really.
"I thought it would be good to give something back. It was a decision of the heart."
And when asked about the anger his decision had sparked back home, he said: I love Wales where I was born.
"There's been a bit of a stir-up there. I expected it."
The Oscar-winning actor has never made a secret of his preference for life in the US.
In the 90s, Hopkins became one of the most bankable and famous faces in films and for the past 20 years he forged his career in the US.
As Nixon he was cast as one of the most famous Americans of the 20th century.
But as Hollywood made him one of their own, his connections with Wales appeared to grow stronger.
Sir Anthony gave £1m to save Snowdon for the nation, and played a big part in promoting Wales abroad.
But many residents of his home town of Port Talbot - where Sir Anthony was the son of the local baker - have accused him of forgetting his roots.
Resident Glanville Mills, 67, said: "Anthony Hopkins is a local boy - it is disgusting what he has done.
"He has lost touch with his roots and should be ashamed of himself."
But the strongest response came from Assembly Member Gwenda Thomas.
She is calling for the Freedom of the Borough of Port Talbot - granted to Sir Anthony four years ago - to be removed.