Daniel Radcliffe, the “Harry Potter” star who made his Broadway debut in Thursday’s Rialto revival of “Equus,” wasn’t nervous about opening night until his costumer told him to be.

He’d been in previews since early September and therefore believed his Broadway debut had already happened. But then he learned backstage that Gotham legiters place prime importance on opening night.

“My costumer said, ‘No no, opening is the night that counts,’ ” Radcliffe said at the apres party at Chelsea Piers. “So it was a bit more nerve-wracking than I expected.”

The omnipresent Harry Potter-gets-a-nude-scene chatter has helped make “Equus” a big seller on Broadway, just as it was in its original London staging last year. And both Radcliffe and co-star Richard Griffiths (“The History Boys”) said they’re enjoying revisiting the psychodrama Stateside.

“I always think that in America there’s more awareness of things psychological,” Griffiths said.

Radcliffe digs it when he gets laughs for a scene in which his character takes a date to a skin pic.

“To get applause for going to porno movie — in this country!” he marveled. “It’s amazing.”

— Gordon Cox