She loves her husband’s killer — as only a mother could.

When the trial of alleged Ivy League murderer Thomas Gilbert Jr. opens in Manhattan this month, the prosecution’s star witness will be his own mom, who told The Post in an exclusive interview that she knows her privileged son pulled the trigger — but says it was only under the weight of severe mental illness.

“I don’t have any choice. You have to do it,’’ Shelley Gilbert said of testifying against her son, 34-year-old Princeton grad Tommy.

“It was obvious he’d done it,’’ the mom said, speaking at length for the first time since the January 2015 slaying of her 70-year-old hedge-fund husband, Thomas Sr.

“There was no one else around. Most trials you have to prove who did it. That’s not true of this one. What’s cruel is I have to relive it on the stand.’’

The day of the slaying, Gilbert Jr. asked his mom to go out to get him a sandwich — and while she was gone, he blasted his dad in his parents’ bedroom in the family’s $6,000-a-month rented apartment in Turtle Bay.

Prosecutors say the son was furious at his financier father for trimming his $3,000-a-month allowance by $200.

Shelley Gilbert returned to the apartment and found her husband and called 911. After police had arrived, her son called her and nonchalantly suggested that “they get a bite to eat.”

Gilbert Jr. eventually returned to his Chelsea apartment, where he barricaded himself inside and was later arrested.

Shelley Gilbert recalled finding her husband’s body and immediately realizing who had pulled the trigger.

She said she thought to herself, “Tommy, you’re far sicker than we ever knew.”

“That wasn’t Tommy,” she said. “He was always a rational thinker.”

Shelley said her son’s conduct both before and after the murder make it inarguably plain that he was in the throes of a psychotic break.

“He’s not an evil person,’’ the mom said.

But prosecutors say Gilbert Jr., who attended the tony Buckley and Deerfield prep schools before college and summered in the Hamptons, was sane enough to know what he was doing.

He staged the slay scene to look like a suicide, they said — leaving his dad dead on the floor with the .40-caliber Glock he’d just used to shoot him in the older man’s left hand.

Shelley must testify against her son because she has been subpoenaed by the prosecution as a witness.

The mom said her son began his mental descent while at Deerfield and was eventually diagnosed with schizophrenia and medicated while at Princeton.

“We’ve been trying to get him into a hospital for 15 years,” his mother said.

His trial has repeatedly been delayed because of questions about his mental health. He has undergone at least five psychiatric examinations and refused to participate in them.

He also will not cooperate with defense experts and will barely communicate with his own lawyers.

The mother said her son’s already fragile grip on reality has spiraled even further since going to jail.

Gilbert Jr. has suffered severe delusions, complaining that he’s being poisoned by radioactive fallout at Rikers. At one point, he ingested a battery.

Shelley, who attends almost every court appearance, said watching her once-handsome son grow increasingly gaunt and glassy-eyed while in custody has been a recurring heartbreak.

“It’s brutal. Brutal. The whole thing has been brutal,” she said. “I still don’t believe all this has happened.

“Think if we put people with cancer in a courtroom with lawyers, the outrage would be overwhelming,” she said. “Yet that’s what we do with our mentally ill.”

She said her son should get mental health treatment instead of a possible life sentence.

“He doesn’t belong in jail, he belongs in a hospital,” she said.

“There’s no value in sending him upstate. … He’s a sick person, who needs to be in a hospital.

“It’s what my husband would have wanted for him if he was still here.”

Gilbert Jr.’s lawyer, Arnold Levine, didn’t return a request for comment.