This must have been a grave mistake.

Someone abandoned a decades-old tombstone along the West Side Highway, and cops are working Saturday to return it to its rightful resting place.

The stone marker, found in a planter between 52nd and 53rd street, appears to belong to Rose Roy Weiss, who died more than 30 years ago in February 1986, a photo provided by sources to The Post shows. She was 83.

The lower corner of the stone was chipped off, but the engraving that remains calls her “beloved.”

The uprooted gravestone was spotted earlier this week by a keen-eyed pedestrian, the West Side Rag reported.

“On Tuesday, I noticed the corner of a thing sticking out of a planter, with the name Rose on it. I decided to pull it out because most of it was obscured, beneath some garbage and half-buried in soil,” the anonymous tipster told the web site.

Cops recovered the tombstone at about 10:30 a.m. Saturday morning, near the pedestrian path along the highway, after searching along Hudson River Park, sources said.

According to the site, Weiss may have lived in Dade County, Florida, based a death record in the Florida Death Index, 1877-1998, but couldn’t confirm the information with the County Clerk’s office.

Police sources say they are investigating where the tombstone came from and how it ended up on the Upper West Side — and whether it’s real.

“We’re looking into it,” a high-ranking police source told the Post. “If it’s legit it would be nice to return it to a family member.”