She began her acting career as a child star and is best known for "Buffy" and "Harriet the Spy."
Trachtenberg's famous friends and former costars paid tribute to the actor.
After Michelle Trachtenberg's death on Wednesday morning, celebrities and fans are paying tribute.
The NYPD told Business Insider that Trachtenberg had been found dead in a luxury apartment in Manhattan near Central Park. No cause of death has been given, and authorities said they're not treating the death as suspicious.
Trachtenberg, who began acting in commercials at age 3, had a decadeslong career in Hollywood, appearing in memorable films like "Harriet the Spy" and "EuroTrip" and TV series like "Buffy the Vampire Slayer" and "Gossip Girl." As the news of her death spread, her famous friends and former costars shared their reactions and heartfelt messages online.
Rosie O'Donnell said she wished she could've helped Trachtenberg.
Rosie O'Donnell costarred with Trachtenberg in the younger actor's film debut.
Evan Agostini/Liaison
In a statement to Us Weekly, O'Donnell, who played the beloved nanny of Trachtenberg's character in her 1996 film debut "Harriet the Spy," called the younger actor's death "heartbreaking."
"I loved her very much. She struggled the last few years. I wish I could have helped," O'Donnell told the publication.
Michelle Branch shared a photo of Trachtenberg with broken heart emojis.
Michelle Branch reacted to Trachtenberg's death.
Michelle Branch/Instagram
Branch was a musical guest star on the sixth season of "Buffy the Vampire Slayer."
Shawn Ashmore called Trachtenberg an incredible person.
Shawn Ashmore previously dated Trachtenberg.
J.Sciulli/WireImage
Ashmore and Trachtenberg dated in the 2000s. In his post about her death, he called it an "incredible loss."
"Incredibly sad to hear about Michelle's passing. She was an incredible person and I will always remember the years we spent together fondly," he wrote. "She was loving, quirky and would never pass up a law and order SVU marathon:) My condolences to her mother Lana and sister Irene!"
Ed Westwick shared a photo of Trachtenberg as her "Gossip Girl" character.
Ed Westwick starred with Trachtenberg on "Gossip Girl."
Ed Westwick/Instagram
Westwick played reformed bad boy Chuck Bass and Trachtenberg played fan-favorite antagonist Georgina Sparks throughout all six seasons of The CW teen drama "Gossip Girl."
"So sad to hear of the passing of @MichelleTrachtenberg," he wrote in his Instagram story. "Sending prayers."
David Boreanaz sent prayers to Trachtenberg's family.
David Boreanaz and Trachtenberg both starred on "Buffy."
David Boreanaz/Instagram
Boreanaz played the vampire Angel, the longtime love interest of Buffy Summers on "Buffy the Vampire Slayer" and was a series regular on the first three seasons of the show before departing for his own spinoff.
Trachtenberg joined the series as Buffy's sister Dawn Summers in season five.
Kim Cattrall shared a throwback photo of her and Trachtenberg in "Ice Princess."
Kim Cattrall was in the 2005 movie "Ice Princess" with Trachtenberg.
Andrew H. Walker/Getty Images
"Rest in peace sweet Michelle ????" Cattrall captioned the post.
Trachtenberg's "Buffy" costar James Marsters shared a heartfelt message.
James Marsters met Trachtenberg when she joined the cast of "Buffy" in 2000.
Kevin Winter/Getty Images
Marsters, who played Spike in "Buffy the Vampire Slayer," worked closely with Trachtenberg from the time she joined the series in 2000 as a young teen.
"My heart is heavy today. We have lost a beautiful soul. Michelle was fiercely intelligent, howlingly funny, and a very talented person," the actor wrote on Instagram. "She died much too young, and leaves behind scores of people who knew and loved her."
"My heart goes out to her family who are good people, and are suffering the greatest loss anyone could bear. I hope everyone can give them space to heal in this most difficult time. Godspeed Michelle. You are missed," Marsters added.