A Manhattan man driven by greed and rage was convicted Friday of murdering his millionaire father after his parents cut his allowance.

Pampered Princeton grad Thomas Gilbert Jr. sat stone-faced as the foreman read “guilty” to the top count.

After more than two days of deliberations, the jury rejected Gilbert’s insanity defense and found him guilty of three of the four counts against him, including second-degree murder and weapon possession charges. He faces up to life in prison when he’s sentenced Aug. 9 before Justice Melissa Jackson.

On Jan. 4, 2015, the avid surfer showed up unannounced at his parents’ Turtle Bay, pressed a .40 caliber Glock against his dad’s temple and pulled the trigger.

Thomas Gilbert Sr., 70, collapsed on the floor of his bedroom, as his son fled.

Assistant DA Craig Ornter argued during five weeks of testimony that Gilbert, 34, may have suffered from mental illness, but he knew exactly what he was doing when he shot his dad.

The morning of the murder Thomas Gilbert Sr. had slashed his son’s allowance following several previous reductions, and his son was enraged.

Gilbert had failed to find steady employment after graduating from Princeton with a degree in economics, instead living off his parents’ largess.

They bankrolled his lifestyle of leisure, paying for international surfing trips, country club memberships, a Chelsea rental apartment and a Jeep.

When his father threatened to take all that away, Gilbert retaliated. He showed up at their Beekman Place apartment, sent his mom, Shelley Gilbert, to get him a coke and sandwich then blasted his dad in the head.

“The last thing Thomas Gilbert Sr. ever saw was his own son pressing a .40-caliber semiautomatic pistol into his head and pulling the trigger,” Ortner said.

Five minutes later, his mother returned to discover her husband’s body on their bedroom floor in a pool of blood.

Defense lawyer Arnold Levine told jurors that Gilbert had suffered from schizophrenia for more than a decade and was unable to maintain a job. It was psychosis and paranoia, not greed that spurred him to kill his father, Levine argued.

“Tommy didn’t understand or appreciate the consequences of his actions,” the attorney said, urging the panel to find Gilbert not guilty by reason of mental defect.

His mother paid the tab for his private lawyer and has repeatedly said her son belongs in a hospital, not prison. Shelley Gilbert was the prosecution’s reluctant star witness, testifying against her son after she was subpoenaed.

In a 911 call played at trial, the operator asks the woman who shot her husband and she replies, “My son, who is nuts. But I had no idea he was this nuts.”

Photos of Gilbert taken just before the murder reveal a strapping blond surfer, with model good looks. A string of ex-girlfriends who testified at his trial said his behavior was strange but he was so handsome they overlooked it.

Since his incarceration, he has unraveled mentally and physically, often babbling incoherently in court with his oversized shirts and slacks hanging off his frail frame.

Shelley Gilbert, who wasn’t in court for the verdict, previously told The Post in an exclusive interview that her son wasn’t evil. “He’s a sick person, who needs to be in a hospital,” she said. “It’s what my husband would have wanted for him if he was still here.”