Ed Westwick, James Marsters, more

Hollywood is in mourning.

Michelle Trachtenberg’s former costars 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.

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.

‘Harriet the Spy,’ ‘Gossip Girl’ actress Michelle Trachtenberg dead at 39

See their reactions below.

Ed Westwick

Trachtenberg’s “Gossip Girl” costar 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

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

Rosie O’Donnell, who starred alongside Trachtenberg in the starlet’s first movie, “Harriet the Spy,” called the actress’s 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.”

David Boreanaz

“So very sad …horrible news,” Trachtenberg’s “Buffy the Vampire Slayer” costar David Boreanaz, 55, wrote on his Instagram story Wednesday. “RIP and prayers to her and her family,” he added.

Kim Cattrall