Michelle Trachtenberg’s untimely death at 39 has sent shockwaves through her fans and colleagues — and her former “Gossip Girl” costar Ed Westwick has posted a tribute to the star.

In a heartfelt post on his IG Story Wednesday, Ed shared a pic of Michelle as Georgina Sparks from the popular teen drama series … captioning it, “So sad to hear of the passing of @michelletrachtenberg.”He added “sending prayers” with the prayer hand and a white heart emoji.

TMZ confirmed the shocking news … with sources telling us police and paramedics responded to a call for a woman in cardiac arrest and found Michelle unresponsive — she had been found by her mother.Michelle was pronounced dead on the scene. The exact cause of her death remains unclear.

A separate source told us Michelle had recently received a liver transplant, and at this point, police reportedly believe she died of natural causes. We’re told her body may have rejected the liver transplant within the last year, causing complications.

Michelle made a major name for herself in Hollywood with standout roles, from “Harriet the Spy” to “Buffy the Vampire Slayer,” solidifying her place as a beloved star.