The mob turned taxpayer-backed Bronx-Lebanon Hospital expansion into its own piggy bank.

Construction expenses at the hospital’s new nine-story outpatient center ballooned by some $5 million — with cash allegedly ending up in the pockets of the Lucchese crime family and hospital executives, The Post has learned.

Lucchese underboss Steven “Wonder Boy” Crea Sr. and associate Joseph Venice were charged with wire and mail fraud in connection with the project at “a major New York City hospital,” according to a federal indictment unsealed last week in a major mob takedown. But the document didn’t identify the hospital or reveal the scheme’s dirty details.

No Bronx-Lebanon executives were named in the indictment, but the federal probe, which began four years ago, is ongoing, and focused on hospital honchos whose palms may have been greased.

Crea, 69, had close ties to Sparrow Construction, the Bronx firm in charge of building the $42 million annex at Bronx-Lebanon. He was a regular visitor to the firm’s offices while the center was under construction, a source told The Post.

Crea worked for Sparrow before he was busted in a 2000 state racketeering case. At that time, he was considered the acting boss of the Luccheses.

Work began on the outpatient center in 2009 and was supposed to take 19 months. But the Health and Wellness Center wasn’t finished until 2014 and was plagued with cost overruns.

Sparrow was the general contractor and billed the hospital $26 million for only $21 million worth of work, sources told The Post.

The heating and ventilation system cost $2.3 million to install. Yet “the hospital still paid somebody $5 million” for it, the source said.

The alleged scheme was carried out through falsified invoices and change orders, the source said.

“The hospital didn’t question one change order,” the source said.

The bulk of the project was paid for through the sale of $36 million in state Dormitory Authority bonds. The hospital is paying back the Dormitory Authority over 25 years.

The 642-bed hospital serves mostly low-income patients. It nearly declared bankruptcy in the 1970s but the state bailed it out.

It receives about 60 percent of its revenue from the state Medicaid program, which amounted to $311.6 million last year, according to the state Health Department.

Yet it pays its execs handsomely.

Longtime CEO Miguel Fuentes’ total compensation came to $1.7 million in 2015, according to its latest tax filings. Fuentes lives a luxury lifestyle with a condominium on the Upper East Side and a Southampton retreat with a pool.

“We have no information whatsoever regarding the recently unsealed indictment,” a hospital spokesman told The Post.

Randy Silverstein, the head of Sparrow Construction, did not ­return a call seeking comment.