If you build it, they will slum.

Like the swallows to Capistrano, junkies are again flocking to West 32nd Street near Penn Station now that the city Department of Transportation has reinstalled a controversial pedestrian plaza there, according to locals and advocates.

“It’s become a hobo jungle,” said Guardian Angels founder Curtis Sliwa, who said he personally has observed people “shooting up right into their leg, where their ankle is” along the stretch.

The Post counted about a dozen apparent vagrants in the plaza on a recent weekday, many seemingly passed out on concrete blocks. Some appeared to be smoking pot; none was observed shooting up.

“It’s bad out here. It’s like zombieland!” said an MTA bus driver.

While the homeless are more visible when the weather is warm, an area worker said there weren’t nearly as many vagrants last year, when the plaza was not open.

“Last summer, it wasn’t so bad,” said the worker, Lonzo S. “I used to have my pizza [there] on the rocks [where the junkies sit], but now I can’t do that anymore.”

The DOT first installed a pedestrian plaza and benches along the strip between Sixth and Seventh avenues at the behest of a major area, landlord Vornado Realty Trust, under a 2015 pilot program.

The plaza almost immediately became a haven for the vagrants.

When the city reinstalled the plaza this year, it replaced the benches with less-than-inviting stone blocks, but that hasn’t stopped the destitute from gathering there.

“The homeless and the emotionally disturbed and the junkies have figured out how to lay out in the rocks,” Sliwa said. “They went to the expense of putting all this hostile architecture … and yet the junkies have adapted.”

There is no law against homeless occupying public spaces.

A courtyard in front of the nearby Church of St. Francis of Assisi across the street — which flies a sign heralding, “All are welcome” — remained empty when The Post swung by last week.

Over the five hours The Post was on the scene, a reporter counted zero homeless-services workers from the city.

The Mayor’s Office said homeless-services contractors canvass the area “multiple times daily” and have placed 15 people “from this area to transitional or permanent housing.”

The Department of Health and Vornado did not respond to requests for comment. The DOT did not provide a comment.

Additional reporting by David Meyer