The King Rat wants out of his cage.

Lawyers for ex-Bonanno crime boss turned FBI informant Joseph Massino — the highest ranking mafioso ever to turn government witness — want the sickly made man sprung from jail as a reward for his devastating testimony against the mafia, his lawyers argued in a Brooklyn federal court filing.

Massino started singing after being convicted of eight gangland murders in 2004, and provided key testimony that sunk dozens of former colleagues including his successor, Vincent “Vinny Gorgeous” Basciano.

The portly don — who will be resentenced tomorrow — dished to the feds that Basciano was plotting to kill Assistant U.S. Attorney Greg Andres and taped the mob big admitting to the murder of family associate Randolph Pizzolo.

“It is fair to say that not only is Mr. Massino the most prominent La Cosa Nostra leader ever to cooperate but also that seldom in the history of the war against organized crime has anyone contributed as much to the government,” wrote attorney Edward McDonald to the court.

The 70-year-old career criminal — who is facing life behind bars — wants to get out on time served.

The letter to Judge Nicholas Garaufis also cites his declining health as a reason to set Massino free.

“Mr. Massino’s long term — and short term — prospects for living are poor,” the missive states.

While he was forced to forfeit $7.5 million in cash and 250 solid gold bars after his conviction, Massino’s attorneys said he would be able to support himself through Social Security and income from some modest rental properties.

Massino’s attorneys also stated in the letter that Massino continues to offer support to ongoing government investigations into the mafia “that have not been revealed yet.”

salgar@nypost.com