A prominent banker was found dead in the tub of his posh downtown apartment with his throat slashed — in what detectives suspect is a suicide, police sources said on Wednesday.

Shawn D. Miller, 42, who traveled the world for Citigroup as one of its top environmental policy experts, sliced his neck and wrists with a knife inside his home at 120 Greenwich St, the sources said.

His body was found by a doorman Tuesday, after his boyfriend called building operators and said he was worried for the banker’s safety.

Miller was last seen on surveillance video getting off an elevator at about 6 p.m. Monday with a mysterious man who was not his beau. The unidentified male was later spotted on camera leaving alone.

Investigators, however, believe the death is a suicide because of the nature of the wounds and the fact a knife was found under his body, sources said.

The doorman who found Miller’s body, Tommy Perez, told investigators that he went to the victim’s apartment on Tuesday afternoon after getting a call from a concerned man who claimed to be the banker’s boyfriend, the sources said.

Miller had built a successful career in assessing the risk of investments.

In 2011, Citigroup promoted Miller to an executive position as managing director of environmental and social risk management.

Miller, who had previously worked for the World Bank, traveled frequently for Citigroup and was instrumental in designing Equator Principles, which banks widely use for establishing voluntary environmental policies.

The bank sent out a note Wednesday informing staff of Miller’s death.

“It is with great sadness that we inform you of the passing of our friend and colleague Shawn Miller,” read the note.

“Shawn joined Citi in 2004 and was highly regarded at Citi and across the financial services industry as a thought leader and tireless advocate for environmental and sustainable business practices. Our heartfelt thoughts go out to Shawn’s family, friends and co-workers during this difficult time. He will be greatly missed by all who knew him.”

The sudden death spooked residents in the building.

Resident Raul Melgoza, 42, said, “I saw it in the news. I just couldn’t believe it happened in the building.

“It’s a pretty quiet building although it’s changed a lot from buyers to more renters. I see a lot of new faces, young college kids.”

Additional reporting by Erin Calabrese and Kevin Dugan