San Francisco’s once-trendy downtown area has descended into a drug-addled hellscape — where addicts regularly overdose in city-funded “dens of death.”

Historic hotels in the Tenderloin neighborhood — which used to be the City by the Bay’s premier entertainment district — are now the face of the progressive California city’s deterioration.

Around 20,000 rooms in about 500 hotels have been converted from coveted tourist destinations into roach- and vermin-infested “Single-Room Occupancy” (SRO) housing for vagrants.

Many of the century-old buildings are now overrun with drug-addled “zombies” high on fentanyl and the flesh-eating animal tranquilizer dubbed “tranq,” residents told The Post during a tour on Tuesday.

“It’s like living in a prison, but worse,” Robert Blackburn said of his squalid room in one of Tenderloin’s SROs.

The neighborhood, located just 2 miles south of tourist hotspot Fisherman’s Wharf, was “once one of San Francisco’s fashionable neighborhoods, home to Bonanza Kings, politicians and millionaire merchants,” according to the San Francisco Chronicle.

Now, Blackburn says he sees overdoses “all the time” as drug dealers run the corners just steps from the squalid hotels.

“I’ve had human feces come up and out of my shower,” Blackburn told The Post. “I try my best to keep my room clean, but there’s been mice [and] lots of roaches in other rooms.”

Longtime Tenderloin resident JJ Smith, who lives near four of the city-funded buildings, said that “once they put these people in these SROs, it’s like they are stuck.”

“The biggest issue is there are too many deaths in and out of there. On a daily basis, I see five overdoses, at least one or two of them end up dead,” he said.

“The only way they leave there is in a coroner’s bag.”

Smith estimates that he’s personally helped revive at least 50 SRO residents in the past year, sharing graphic video of one such harrowing scene from June.

The shocking video shows the woman unconscious on the ground of the filthy room as Smith passes Narcan to the boyfriend to try to revive her.

He squirts the Narcan — a drug used to reverse overdoses — into the woman’s nostrils, but she doesn’t move.

“Wake up, girl,” the boyfriend says. “Come on, baby girl! Don’t die on us!”

Smith repeatedly calls out her name, while her boyfriend shakes her repeatedly in a desperate effort to wake her up.

“She survived that time, but she died about a month later inside the SRO,” Smith told The Post.

“I saw her body be wheeled out by the coroner. She overdosed and died because no one was there to help her that time.”

The city runs nearly 20,000 rooms in about 500 SRO hotels, according to San Francisco’s Department of Building Inspections 2021-22 stats. The rooms are typically 8 by 10 feet and the residents share a common bathroom on each floor.

Many of the residents in the city’s hundreds of SROs are vagrants who first lived in encampments or one of the homeless shelters before securing a room. Some are recovering addicts, but many “fall off the wagon” and start using drugs again while living in the SRO, which is against the rules each resident must follow.

Blackburn, a recovering heroin addict who is now taking methadone, said the SRO desk managers who man the doors and check on residents “don’t do much” to push them into seeking treatment and counseling.

“They don’t push it because they can’t control us, and really, it’s not their job,” Blackburn said.

The SROs were not meant to be used as permanent housing, but many of the recovering and still-struggling addicts stay in the roach- and mice-infested rooms for years.

“We need help,” Blackburn said. “We keep asking, but our voices aren’t being heard.”

Block after block in the Tenderloin district, drug dealers can be seen handing balled-up foil to addicts on the street.

Business owners said they are frustrated with the amount of homelessness and drug overdoses happening right in front of their doors.

Residents and business leaders alike said the blight surrounding Tenderloin and other areas in downtown have contributed to the city’s “doom loop” and flagship businesses like Nordstroms and Whole Foods to leave the area.

“A lot of people just don’t want to come down to this neighborhood anymore,” said Tommy, who owns a Vietnamese and Chinese restaurant on Larkin Street.

“We had the grand opening 10 days right before the [COVID-19] pandemic, but business is so slow. People see the street conditions, the homeless. I hope this neighborhood will get better.”

Workers from local non-profits paid by the city come to the Tenderloin to power wash the streets where families and young students walk daily. But right after the sidewalks dry up, the homeless addicts go right back to squatting on the street.

On Monday night, a homeless man allegedly holding a knife and threatening residents at the 300 block of Jones Street was shot by San Francisco Police. The unidentified man was rushed to a local hospital with life-threatening injuries, police officials said.

The next morning, a group of homeless men sat a few blocks away from where the shooting occurred and hot-wired one of the city’s electric poles, which they used to operate a TV in one of their tents.

“If they are smart enough to do that, I want to get them into some program because they can get a job where they can actually use that skill,” said one former city worker who wanted to remain anonymous.

City leaders said their hands are tied because of an injunction banning San Francisco Police from clearing most homeless encampments.

A judge ordered the injunction while a lawsuit filed by the Coalition on Homeless — which claims the city has violated state and federal laws on homeless individuals’ rights — is ongoing.

“It is not humane to let people live on our streets in tents,” said Mayor London Breed last week. “We want a reversal of this injunction that makes it impossible to do our jobs.”

Meanwhile, residents of the once-fashionable district say that while there are services available to them provided by non-profits and paid by the city, nothing is being done to push them into drug rehabilitation treatments — and they are ultimately left behind.

Overdoses from fentanyl claimed 62 lives out of 71 total deaths from drug overdoses last month, according to statistics released by the Medical Examiner’s Office. The grim figure now places San Francisco on track to break a 2020 record for the total number of overdoses, when 712 people died.