A pair staying at a Hell’s Kitchen hotel left their housekeeper an ominous note and a cash tip — before they killed themselves inside their room, police sources told The Post on Friday.

“Don’t look behind the curtain. We’re dead. This is for you,” the two wrote on a note taped to a sheet in front of the doorway, along with some twenty-dollar bills, sources said.

But the cleaner for Yotel New York ignored the message and discovered them dead at around 7 p.m. Thursday lying face-up in a bed with a large, single, clear plastic bag over both their heads in an 18th-floor room of Tenth Avenue building, according to sources.

Wedged into the bag were two plastic tubes connected to a pair of red canisters of nitrous oxide, according to sources.

Found under the hotel bed was the book “Suicide and Attempted Suicide: Methods and Consequences” by Geo Stone, which was bookmarked to the section on asphyxiation, sources said.

When the cleaner first entered room No. 1830, she found the sheet hung in front of the doorway, blocking the view of the room, before finding the tragic double-suicide scene, sources said. The note and gratuity were taped to the sheet.

A suicide note signed by the pair explained that both had intended to kill themselves, sources said.

Two large envelopes also were left in the room containing their wills, according to sources.

Emergency responders pronounced the two, whose names were not officially released, dead at the scene.

A city Medical Examiner’s Office investigator deemed the incident consistent with strategically planned suicides, sources said.

On Friday, guests were seen checking in and out of rooms on the 18th floor of Yotel, close to where the suicides took place.

“It happens all the time every day on the streets, you know,” said an 18th-floor Yotel cleaner who would only identify herself by the name of Gracie. “It’s never happened [at Yotel] before.”

Neighbors of the woman who took her life were stunned by the matter.

“Wow, oh my goodness,” said Carmen Arsty, 60, who noted that the woman was married and had recently moved.

The relationship between the two was not immediately clear.

The double suicide is reminiscent of a June 2013 case in which a Park Slope couple who hosted a self-help radio show committed suicide in their home by suffocating themselves with helium and plastic bags.

Psychotherapist Lynne Rosen, 46, and her motivational-speaker husband John Littig, 48, left behind suicide notes detailing Rosen’s battle with bipolar disorder, police sources told The Post at the time.

Additional reporting by Elizabeth Rosner and Stephanie Pagones