A young man was fighting for his life after going over the railing of an elevated park near the South Street Seaport, authorities said Tuesday.

The unidentified man fell off the first-floor green space, known as the Elevated Acre, at 55 Water St. just after 1:33 p.m., officials said.

Edwin Tejada, 38, a worker at the Impark garage under the park, said he talked to a customer who witnessed the horror.

“I was driving in, and the lady said, ‘Oh, my god, he jumped!’ ” Tejada said. “He almost fell on her. I heard the boom and looked back.”

Tejada said the victim wasn’t moving after he landed on the ground, and authorities said EMS rushed him to Bellevue Hospital in critical condition.

Witnesses said he appeared to be in his early 20s, with a white t-shirt, black striped gym shorts and white sneakers.