Mara Wilson
"My heart is breaking for my childhood friend," the Matilda alum wrote on Instagram Story . "Your memory will be a blessing, Michelle."
Melissa Gilbert
"Oh Michy," Melissa wrote on Instagram in tribute to her A Holiday for Love costar. "And we lived so close to one another…..my heart aches for your family and all those who loved you so… @michelletrachtenberg #ripsweetgirl"
Ed Westwick
"So sad to hear of the passing of @michelletrachtenberg," the Gossip Girl alum wrote on his Feb. 25 Instagram Story. "Sending prayers."
He closed his message with the praying hands and white heart emojis.
David Boreanaz
"So very sad..." Michelle's Buffy The Vampire Slayer costar wrote on social media of her passing. "Horrible News. RIP and prayers to her and her family."
Kevin Zegers
The Gossip Girl alum—who played Damien Dalgaard in the teen drama—described Michelle's passing a "loss."
"Michelle was tough and a force," he added in his Instagram post. "Just a kid, so sad."
J. August Richards
The Angel alum offered his support to those closest to the actress, writing on Instgram, "My heart goes out to her family and loved ones. What a beautiful spirit who left an amazing legacy… Rest In Peace, dear one."
Beanie Feldstein
"I'm so beyond heartbroken," the Booksmart star wrote on Instagram Story, adding that Michelle's performance as the titular kid detective in Harriet the Spy "raised all of us little girls wanted to BE something."
Chris Colfer
“Michelle was the absolute sweetest and one of the most supportive people I knew," the Glee alum wrote on Instagram. "I’m so lucky I got to be her friend."
Iris Apatow
“I watched her through every stage of my life,” the actress, who is the daughter of Michelle's 17 Again costar Leslie Mann, wrote on Instagram Story. “She brought me a lot of joy when things were hard—I’m so heartbroken. Rest in peace.”