Michelle Trachtenberg, a former child star who appeared in the 1996 “Harriet the Spy” hit movie and went on to co-star in two buzzy millennial-era TV shows — “Buffy the Vampire Slayer” and “Gossip Girl” — has died. She was 39.

Police responded to a 911 call shortly after 8 a.m. at a luxury residential tower in midtown where “officers observed a 39-year-old female unconscious and unresponsive,” according to an NYPD statement.

Paramedics pronounced her dead at the scene. No foul play was suspected and the New York Medical Examiner is investigating the cause of death, police said.

Representatives for Trachtenberg did not immediately respond to a request for comment. Her death was first reported by the New York Post.

Trachtenberg was raised in Brooklyn's Sheepshead Bay neighborhood.

Trachtenberg was 8 when she began played Nona Mecklenberg on Nickelodeon’s “The Adventures of Pete & Pete Nona” from 1994 to 1996 and then starred in the title role in the film adaptations of “Harriet the Spy” and “Inspector Gadget,” opposite Matthew Broderick.

“Michelle comes off as genuine because she really is a genuine kid. Everyone can identify with her,” said Debby Beece, president of Nickelodeon Movies in 1996.

In 2000 Trachtenberg joined the cast of “Buffy,” playing Dawn Summers, the younger sister of the title character played by Sarah Michelle Gellar between 2000 and 2003.

Trachtenberg thanked Gellar for speaking out against Joss Whedon in 2021, following abuse allegations made against the “Buffy” showrunner. “I am brave enough now as a 35-year-old woman to repost this,” she wrote on social media, and alluded to “his not appropriate behavior” she experienced as a teenage actor.

In 2001, she received a Daytime Emmy nomination for hosting Discovery’s “Truth or Scare.” Trachtenberg went on to recurring roles on “Six Feet Under,” “Weeds” and “Gossip Girl,” where she played the gang’s scheming nemesis, Georgina Sparks.

Her other credits included “Ice Princess” in 2005 — playing a math prodigy and aspiring figure skater — and the 2004 teen sex comedy “EuroTrip.” For “Killing Kennedy,” the 2013 film in which she played the wife of Lee Harvey Oswald, around 80% of Trachtenberg’s dialogue was in Russian. She had learned the language from her mother growing up.