The young woman who plunged to her death from a posh building in lower Manhattan this weekend was acting strangely before she leapt from the fourth-floor window, sources told The Post on Sunday.

Cops have not released the identity of the 23-year-old woman, pending her family’s notification, but said she was visiting New York City from elsewhere when she died tragically Saturday.

A source with knowledge of the incident said the woman stopped by the Bortolami art gallery that occupies the first and second floor of the five-story building on Walker Street in Tribeca.

Instead of staying in the gallery, however, she walked up the steps to the fourth floor and flung herself out the window into the backyard, according to the source.

“She was acting erratic,” the source told The Post. “She ripped the blinds down, opened the window and jumped out.”

Police found the woman unconscious shortly after 5 p.m. at 39 Walker Street.

Authorities brought her to Bellevue Hospital in critical condition, where she was later pronounced dead.

Several neighbors said they saw police, firefighters and emergency medical services at the scene. But few knew what happened.

Saturday is typically the gallery’s busiest day, the source said.

“When the gallery is open, they open the doors to the hallway” so one can walk up the steps from first to second floor, the source said.

Visitors, however, “are not allowed up” on the fourth floor, which houses apartments — with 2 bedroom units listed in the $2.5 million range.

“I couldn’t sleep last night because I kept thinking of someone jumping,” the source added. “I’m still in shock right now.”

 If you are struggling with suicidal thoughts or are experiencing a mental health crisis and live in New York City, you can call 1-888-NYC-WELL for free and confidential crisis counseling. If you live outside the five boroughs, you can dial the 24/7 National Suicide Prevention hotline at 1-800-273-8255 or go to SuicidePreventionLifeline.org.