Barnard College freshman Tessa Majors was allegedly in Morningside Park to buy marijuana when she was fatally stabbed by a group of teenage robbers, the head of the NYPD sergeants’ union claimed Sunday.

“What I am understanding is that [Majors] was in the park to buy marijuana,” Sergeants Benevolent Association president Ed Mullins told radio host John Catsimatidis on his AM 970 show, “The Cats Roundtable.”

“Here we have a student murdered by a 13-year-old, we have a common denominator: marijuana,” said Mullins, referring to the 18-year-old Majors and one of her alleged killers, 13-year-old Zyairr Davis, who neighbors have said smokes weed.

Police sources confirmed to The Post that they are investigating the claim that Majors was in the park to score weed, a detail that came from a college friend of the victim.

The pal claimed to detectives that the 18-year-old Virginia native and musician said she was headed to the Upper Manhattan park to buy pot Wednesday evening, according to sources.

Majors was later found fatally stabbed on stairs near West 116th Street and Morningside Drive. Her bag was gone.

A Columbia University security guard found Majors lying just outside Morningside Park around 5:30 p.m. Wednesday, bloodied from stab wounds to her face, neck and arms, according to authorities and sources.

Davis was arrested Thursday and allegedly told detectives he and two middle-school buddies robbed her before one of the boys knifed her, sources said. It wasn’t clear what sparked the fatal knifing or whether the boys were involved in any drug deal that went down or one that was supposed to.

Despite purportedly admitting a role in the mugging, Davis has denied plunging the knife into Majors.

Another youth, 14, was arrested but ultimately released Saturday as authorities worked to bolster their case.

A third teen — the middle schooler accused of actually doing the stabbing — is still being sought.

Mullins said Sunday that lax police enforcement, both with respect to marijuana and in general, has the city trending in the wrong direction.

“We don’t enforce marijuana laws anymore. We’re basically hands-off on the enforcement of marijuana,” Mullins said.

“I understand the mayor made statements that this is surprising on how this can happen in New York City,” he said. “I really have to question what world he’s living in to think that this is surprising, when we are watching the city slowly erode, with shootings, stabbings, an increase in homicides and, most importantly, a hands-off policing policy.

“Something needs to change and it needs to change quickly or it’s gonna be very difficult to put the genie back in the bottle.”