In a statement to Us Weekly, Rosie 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.

Alyson Hannigan, who played Willow Rosenberg in "Buffy the Vampire Slayer," shared a series of photos of herself and Trachtenberg on Instagram. Trachtenberg joined the series as Buffy's sister, Dawn Summers, in season five.

"I am deeply saddened by the news of Michelle's passing. She brought a loving energy to the set of Buffy. My thoughts are with Michelle's family and friends," Hannigan wrote in her caption.

"I'm so sorry your bright light died so young," Emma Caufield, who played Anya on "Buffy the Vampire Slayer," wrote in her Instagram story caption. "Our Buffy family lost a little sister today... Rest in peace lovely Mish Mish. You were loved."

James Marsters, who played Spike in "Buffy the Vampire Slayer," worked closely with Trachtenberg from the time she joined the series as a teen in 2000.

"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.

David Boreanaz played the vampire Angel, Buffy Summers's longtime love interest, on "Buffy the Vampire Slayer." He was a series regular on the first three seasons of the show before departing for his own spinoff.

Michelle Branch was a musical guest star on the sixth season of "Buffy the Vampire Slayer."

Blake Lively, who played Serena van der Woodsen in The CW teen drama "Gossip Girl," shared a photo of herself and Trachtenberg from the show on her Instagram story. Trachtenberg played fan-favorite antagonist Georgina Sparks in the show.

"This was the first day I met Michelle. She was electricity. You knew when she entered a room because the vibration changed. Everything she did, she did 200%," Lively wrote in her caption.

The "It Ends With Us" actor also recounted how Trachtenberg would always have "yummy caramel smelling lip gloss" on because "she didn't just want to sparkle on camera, she liked creating a nice experience for anyone who was in her orbit, even down to the subtle smell of her lipgloss because she cared about the sweet details."

Lively then called Trachtenberg "a kind person, through and through."

"Hold those you love and have loved dear. The world lost a deeply sensitive and good person in Michelle. May her work and her huge heart be remembered by those who were lucky enough to experience her fire," Lively wrote.

Ed Westwick played reformed bad boy Chuck Bass throughout all six seasons of "Gossip Girl."

"So sad to hear of the passing of @MichelleTrachtenberg," he wrote in his Instagram story. "Sending prayers."

Chace Crawford played Nate Archibald on "Gossip Girl."

"Michelle was one of a kind. I remember her coming on set for the first time and just absolutely owning it. She was a force of nature and just so so unapologetically funny and magnetic.. remembering those years with a big smile. Just a terrible loss. Love you," Crawford wrote in his Instagram caption.

Shawn 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!"

Kim Cattrall and Trachtenberg acted together in the movie "Ice Princess" in 2005.

"Rest in peace sweet Michelle 💔" Cattrall captioned the post.

Fellow child star Mara Wilson posted a photo of Trachtenberg on her Instagram story.

"My heart is breaking for my childhood friend. Your memory will be a blessing Michelle," Wilson wrote in her caption.

"My heart aches for Michelle Trachtenberg. We grew up in NY audition rooms together and our moms were friends. We reconnected in LA during our messy young Hollywood era. It was like no time passed. Her sassiness always made me laugh," Christy Carlson Romano wrote on her Instagram story.

The actor said she "wished" they had stayed in touch.

"I never got to tell her that years before we reconnected I wrote and recorded this song for her movie 'Ice Princess.' It never made the cut, but every time I thought of her it would go through my mind and now it will forever. Rest in peace," she wrote.

Kate Beckinsale posted an old paparazzi photo of herself and Trachtenberg on her Instagram story.

"What a terrible shock and loss, my god. My heart aches for your mama. Fly safe little bird," Beckinsale wrote in her caption.