Hollywood is in mourning.

Michelle Trachtenberg’s former co-stars and friends are reacting to the actress’s sudden death at just 39.

The Post broke the news that Trachtenberg, known for her roles on “Buffy the Vampire Slayer” and “Gossip Girl,” was found by her mother around 8 a.m. Wednesday at her luxury apartment complex in Manhattan’s Central Park South neighborhood.

“It is with great sadness to confirm that Michelle Trachtenberg has passed away. The family requests privacy for their loss. There are no further details at this time,” her rep Gary Mantoosh told The Post in a statement.

The “Gossip Girl” star underwent a liver transplant within the last year. Per sources, it’s believed her body may have rejected the liver transplant and that Trachtenberg died of natural causes.

Ed Westwick, James Marsters, David Boreanz and Rosie O’Donnell were among the celebrities paying tribute to Trachtenberg on Wednesday, with heartfelt messages.

See their reactions below.

Trachtenberg’s “Gossip Girl” co-star Ed Westwick shared his reaction to the star’s death on his Instagram Story.

“So sad to hear of the passing of @michelletrachtenberg,” Westwick, 37, wrote on Wednesday alongside a photo of Trachtenberg on “Gossip Girl.”

“Sending prayers,” he added.

James Marsters, who played Spike on “Buffy the Vampire Slayer,” told The Post, “My heart is heavy today. We have lost a beautiful soul.”

“Michelle was fiercely intelligent, howlingly funny and a very talented person,” he continued. “She died much too young, and leaves behind scores of people who knew and loved her.”

Marsters, 62, added, “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,” the actor concluded.

Rosie O’Donnell, who starred alongside Trachtenberg in the starlet’s first movie, the Nickelodeon-produced 1996 film adaptation of the children’s novel “Harriet the Spy,” called the actress’ death “heartbreaking.”

“I loved her very much,” O’Donnell, 62, said in a statement to Us Weekly on Wednesday. “She struggled the last few years. I wish I could have helped.”

“So very sad … horrible news,” Trachtenberg’s “Buffy the Vampire Slayer” co-star David Boreanaz, 55, wrote on his Instagram Story Wednesday. “RIP and prayers to her and her family.”

Alyson Hannigan, who played Willow on “Buffy,” wrote on Instagram: “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, 50, concluded her post, which included photos of her and Trachtenberg over the years.

Kim Cattrall, who starred alongside Trachtenberg in the 2005 Disney film adaptation of author Meg Cabot’s book “Ice Princess,” mourned her co-star on social media.

“This is Heartbreaking. 💔 So talented, much too young,” she commented under a news post on X about Trachtenberg’s passing. “RIP Sweet Beauty. xo.”

The “Sex and the City” star also shared a tribute to Trachtenberg on Instagram.

“Rest in peace sweet Michelle,” she captioned a snap of her and the “Gossip Girl” alum from the film.

“SNL” star Kenan Thompson shared a heartfelt Instagram post mourning Trachtenberg, who he knew when they both started out as child actors on Nickelodeon.

“Our first Nick movie star has departed us!!” Thompson, 46, captioned his post on Wednesday. “She was my friend and now she rests!! 🙏🏾.”

He added, “Check on your people!!! ❤️.”

Thompson’s Instagram post included a video of him and Trachtenberg from the 1999 Nickelodeon Kids’ Choice Awards in which she acted as a commentator on a “mega mini golf” competition between Thompson and Randy “Macho Man” Savage.

Mara Wilson, who like Trachtenberg achieved fame as a child star, remembered the “Gossip Girl” alum on her Instagram Story Wednesday.

“My heart is breaking for my childhood friend,” Wilson wrote alongside a photo of Trachtenberg. “Your memory will be a blessing, Michelle.”

The “Matilda” star also reposted a tribute shared by her and Trachtenberg’s mutual “childhood friend,” singer-songwriter and former child actor Zane Carney.

“A beautiful tribute to a dear childhood friend, from another dear childhood friend,” Wilson captioned the repost.

“Clarissa Explains It All” star Melissa Joan Hart posted a scene of her and Trachtenberg on the ’90s teen sitcom on Instagram, writing, “I’m heartbroken to hear of the passing of @michelletrachtenberg!! So young, so talented and so sweet! Here is a fun scene from when she was on #ClarissaExplainsItAll.”

She added, “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.”