Alyson Hannigan

Chace Crawford

James Marsters

"My heart is heavy today," the Buffy alum wrote on Instagram. "We have lost a beautiful soul. Michelle was fiercely intelligent, howlingly funny, and a very talented person. She died much too young, and leaves behind scores of people who knew and loved her."

James continued, "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."

Kate Beckinsale

Blake Lively

"She was electricity," the Gossip Girl alum wrote on Instagram Story. "You knew when she entered a room because the vibration changed. Everything she did, she did 200%. She laughed the fullest at someone’s joked, she faced authority head on when she felt something was wrong, she cared deeply about her work, she was proud to be a part of this community and industry as painful as it could be sometimes, she was fiercely loyal to her friends and brave for those she loved, she was big and bold and distinctly herself."

Blake also remembered Michelle's " yummy caramel smelling lipgloss," noting that the late star wore it because "she liked creating a nice experience for anyone who was in her orbit."

"The world lost a deeply sensitive and good person in Michelle," she added. "May her work and her huge heart be remembered by those who were lucky enough to experience her fire."

Charisma Carpenter

"It is never easy to see someone so young, bright, beautiful, and full of promise pass away," the Buffy alum wrote on Instagram. "It’s devastating, actually."

"I would be remiss not to acknowledge the undeniable impact she has had on the lives of her fans worldwide," she continued. "She will live on through her performances and her love of storytelling forever."

The actress, who played Cordelia in the teen drama, concluded, "Rest easy, Michelle."

Leslie Mann

Christian Siriano

Kim Cattrall

Clare Kramer

Kenan Thompson

Rosie O'Donnell

Emma Caulfield

Matt McGorry

"She had a fierceness but also a deep caring and softness with those she trusted," the Orange Is the New Black actor wrote on Instagram. "She always stood up for her friends, was incredibly talented and had the darkest sense of humor. She had the spirit of a true New Yorker."

Though the two were "really close but lost touch for quite a while," Matt said he was "always sure that we’d cross paths again."

"I was looking forward to meeting as the people we’d grown to become," he added. "You will be missed by so many. Love you, Michelle."

Kristin Chenoweth

Shawn Ashmore

Michelle's ex-boyfriend, whom she dated between 2004 and 2006, penned a tribute on social media. 

"Incredibly sad to hear about Michelle’s passing," he wrote. "She was an incredible person and I will always remember the years we spent together fondly. She was loving, quirky and would never pass up a law and order SVU marathon:)"

He added, "My condolences to her mother Lana and sister Irene! This is an incredible loss!"

Kimberly J. Brown

"We met on the kid actor audition circuit and had so many fun, unique adventures both in and out of the Biz together, even coming out to act in LA around the same time as young teens," the Halloweentown alum wrote on Instagram. "As older adults we didn’t see eachother as often but our bond over those early days was always there-as was her outgoing, feisty, funny personality and deep love of acting."

She added, "Sending so much love to her mom Lana and her sister Irene whom she loved so much.I love you, Michelle. Rest easy."

Melissa Joan Hart

"I'm heartbroken to hear of the passing of @michelletrachtenberg," Melissa wrote on Instagram, alognside a throwback clip from Clarissa Explains It All. "So young, so talented and so sweet!"

She added of the clip, "I think this was about 1992 and the episode was an idea I had about babysitting a total nightmare. But Michelle was nothing like her character and even this young, we got along wonderfully."

Josh Schwartz & Stephanie Savage

“We are deeply saddened by the news of Michelle’s passing," the Gossip Girl creators and showrunners shared with NBC News. "Michelle was so funny, kind, and talented. Her portrayal of Georgina Sparks was an iconic fan favorite, and grew from a one season villain to a beloved character who returned over the course of six seasons."

The pair continued, "She was a delight to have on set and will be deeply missed. Our thoughts are with her family.” 

Mara Wilson

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

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

Beanie Feldstein

Chris Colfer

Iris Apatow