Former inmates at Rikers Island and other Big Apple lockups have left behind a staggering $4.2 million in their commissary accounts — and it’s still waiting to be refunded, officials say.

The hefty pot of money is linked to more than 319,000 commissary accounts of ex-detainees as of May this year, the city’s the Department of Corrections said in a report on the unclaimed dough.

Some of the accounts may belong to the same inmates because of repeated stints at the jails, but the number of individuals owed money would still be in the thousands.

Details of the unclaimed millions were laid bare by the department under new city legislation that requires jails to publicly release the refundable commissary figure each year.

The DOC, under the 2021 City Council law, also has to detail its annual efforts to return the cash to its former inmates.

The report, first cited by The City, said agency officials notified incarcerated inmates about the leftover dough and how to retrieve it by posting flyers in its various lockups.

“Formerly incarcerated individuals can claim their unused commissary funds in person at Department Cashier Windows or giving the Department an address to which a check can be sent by mail,” the report stated.

“Individuals can immediately receive up to $200 in cash of such funds when they are discharged from the Department’s custody to the community.”

It is unclear what the city may do with any unclaimed funds. At one point, jail officials suggested inmates vote on how to use the money, such as for new gym equipment, The City said.

The Corrections Department didn’t immediately respond to The Post’s request for comment Friday.