Now we know Ralph Lauren’s alter ego. By day, he’s an accomplished fashion designer. But by night ... he’s Aquaman.

That became evident after Monday night’s elaborate presentation of his spring Polo For Women line, which he unveiled in a video projected on a wall of water some five stories high that shot up from Cherry Lake in Central Park.

Yep, you read that right. The runway models in the 11-minute film, and Lauren, who takes a bow at the end, look like holograms and appear to walk on water.

Hundreds of Ralph fans — including top models (Chanel Iman, Karlie Kloss), celebs (singer Ciara, Kendall Jenner) and athletes (Rangers goalie Henrik Lundqvist) — watched what Lauren calls a groundbreaking “4-D” fashion event, held after dark.

Was there a contingency plan if it rained?

“No,” said David Lauren, chuckling. The designer’s son and executive vice president of advertising, marketing and corporate communications was on hand, along with his wife, Lauren Bush Lauren, who noted the pressure’s been on her husband to help pull this off for weeks. And it’s not over yet.

“We’ve still got two more nights to go,” she said.

The video will be shown again Tuesday night to the public, and Wednesday to buyers and retail execs.

Ralph walking on water? That never gets old.