It’s a slithery mystery.

A 5-foot-long snake was trying to get inside an Upper West Side apartment Wednesday morning — and no one’s sure where it came from, cops and the man who found it said. 

Police were called to the building on West 87 Street near Columbus Avenue when resident Sam Sullivan saw the boa constrictor slither on an exterior gate near his basement apartment around 8 a.m.

“There was a snake in between my fence and my neighbor’s fence,” Sullivan, 50, told The Post. 

“It was at the top of the fence, and I called 911.”

The NYPD’s Emergency Service Unit converged on the scene and grabbed the reptile, delicately placing it in a polka-dot-colored pillowcase, according to footage the NYPD released. 

Police then handed the snake over to the Animal Care Centers of NYC.

Cops said it was unclear who owned the brown-spotted boa constrictor and where it originated.

“I have no idea where it came from,” Sullivan said, while calling the discovery an “urban legend.”

“It’s like literally someone had a pet that got loose and ended up on my fence,” he added. 

“I just don’t want PETA thinking I had snakes.”

The freakish find left other locals shaken.

“It’s all super unsettling because I have a cat and a little daughter, and it was a boa constrictor apparently,” resident Alex Noschese, 36, told The Post.

An Animal Care Centers spokesperson said boa constrictors are illegal to own in the Big Apple.  

The creature was placed with a foster guardian outside of New York City, the spokesperson said. 

The discovery came days after a New Yorker crashed a rented U-Haul van when he found a live 3.5-foot white snake under his seat while transporting a couch with his roommate. 

The driver picked it up, thinking it was a toy left behind.

“It was warm and heavy. Then it turned its head and looked at me,” Jared, who didn’t want to give his last name, told The Post after smashing into a car in Soho.

“I hate snakes. I freaked out. I took my foot off the brake and crashed into the car in front of us.”