It’s the city’s secret tragedy: the giant-octopus attack on the Cornelius G. Kolff, a Staten Island Ferry boat dragged to a ­watery grave with 400 souls aboard on Nov. 22, 1963.

Few recall the harbor horror because news coverage was eclipsed by the shocking assassination of John F. Kennedy in Dallas that very day.

Also because it never actually happened.

But truth is no obstacle to artist Joe Reginella, the hoaxster whose slick brochures, Web site and even a statue are luring hapless tourists to a far corner of Staten Island in search of a ­museum devoted to the fantastical fish tale.

“There aren’t really giant octopuses here — are there?” asked Nicole Welsh of Melbourne, Australia, casting a wary eye over the seemingly placid waters of New York Harbor last week.

Glossy fliers touting the nonexistent Staten Island Ferry Disaster Memorial Museum began popping up in downtown Manhattan and on Staten Island three weeks ago. They promise an octopus petting zoo, historical exhibits and a “Ferry Disastore” gift shop. It includes directions to a fictitious shoreline address across the street from the Snug Harbor Cultural Center.

Workers at the center, which houses dozens of cultural groups, have been running into confused visitors wandering around the sprawling grounds looking in vain for the ferry-disaster museum.

“I explain that it’s an ­urban hoax, like the crocodiles in the sewers,” said Nick Dowen of the Noble Maritime Collection, who has been fielding questions from curiosity seekers.

Francesca Navarro, who works the front desk of the Staten Island Museum, another spot the brochure-bearing tourists have been descending upon, said, “I think they maybe have a suspicion it’s fake, but they feel like they just have to prove it.”

Museum spokeswoman Rachel Somma added, “Even we were half-wondering if something really had happened.”

Said Australian tourist Tamara Messina: “The brochure sounded very intriguing,” adding that her three young sons “seemed a bit more concerned that it may happen again” as the family rode the ferry.

The Web site for the “museum” includes an authentic-looking Wikipedia entry, mocked-up newspaper articles, a video “documentary” — and an online shop hawking $25 “Octopus Memorial T-shirts.”

The T-shirts are the only thing that’s actually real.

The Cornelius G. Kolff did ­exist. It was a steam-powered Staten Island Ferry boat for more than 30 years, starting in 1951, and it met its doom not in a sea monster’s maw, but at the city Department of Correction. The agency turned the vessel into a floating Rikers Island inmate dorm in 1987, then sold it off for scrap in 2003.

Reginella, the mastermind behind the hoax, told The Post the prank took six months to plan and that it’s “part practical joke, part multimedia art project, part social experiment.”

He carts his “memorial” statue depicting the tragic event in cast bronze to Battery Park and other public locations and watches the startled reactions of passers-by.

The 45-year-old Staten Island artist, who creates store windows and stage sets as his day job, has a knack for macabre humor. A “Jaws”-themed baby crib he built in 2015 became an Internet hit.

His hoax “is not malicious,” he said, “but I guess some people are agitated about it.” One hoodwinked woman tracked him down and berated him over the phone. “How can you get so bent out of shape?” he said. “It’s a joke.”