A financial fraudster desperate for leniency from the feds tipped off investigators to the massive college-ad missions bribery scandal, then wore a wire to help bring down the cheats, a report said Thursday.

With his back to the wall in an alleged pump-and-dump scheme, Los Angeles financier Morrie Tobin offered up a tip that would turn into the biggest college-admissions fraud the feds have ever prosecuted, according to The Wall Street Journal.

In a bid for mercy, the Yale grad told investigators that Rudolph “Rudy” Meredith, the head women’s soccer coach at the Ivy League school, solicited $450,000 from him in exchange

for recommending Tobin’s daughter for admission as an athletic recruit, a source told the Journal.

Tobin agreed to wear a wire during a meeting in a Boston hotel room with Meredith — who then also began cooperating.

That, in turn, led the feds to admitted mastermind William “Rick” Singer, a California college-prep expert who raked in more than $25 million from wealthy parents to get their underqualified kids into top schools, including Yale, Wake Forest, Georgetown and Stanford.

Through a 10-month investigation dubbed Operation Varsity Blues, investigators learned the scam dated as far back as 2011 and involved dozens of allegedly crooked ACT and SAT proctors, college athletic coaches and parents.

“Fuller House” actress Lori Loughlin, her fashion-designer husband, Mossimo Giannulli, and former “Desperate Housewives” star Felicity Huffman were among the indicted.

Tobin, who was not charged in Operation Varsity Blues, is awaiting sentencing after taking a plea deal in the financial-fraud case, according to the Journal. An FBI spokeswoman from the Boston field office said the overall securities-fraud investigation is still ongoing.

Singer for years ran Edge College & Career Network of Newport Beach, Calif. The feds say he secretly had his well-heeled clients funnel money into a sham charity so he could pay off his cohorts to weasel their kids into school.

One parent, a Silicon Valley investor, said Singer proposed using a “side door” to get his teen into USC under the guise of being a football player — but the proposal left the dad feeling “dirty,” he told Axios.

“[Singer] kept pushing it, but I finally just said to give some other child the opportunity,” said the parent, who hasn’t been approached by law enforcement.

USC said it would conduct a case-by-case review for any current students and graduates who may be connected to the alleged scheme.