Anton Yelchin, who was best known for playing Chekov in the new Star Trek films, has been killed by his own car at his home in Los Angeles, police say.
It struck him after rolling backwards down the steep drive at his Studio City home, pinning him against a brick postbox pillar and a security fence.
He died shortly after 01:00 (08:00 GMT) on Sunday.
JJ Abrams, who directed Yelchin in the first two Star Trek films, paid tribute on Twitter saying he was "brilliant".
"You were kind. You were funny as hell, and supremely talented. And you weren't here nearly long enough. Missing you," he wrote.
Yelchin played Pavel Chekov, the role originally made famous by Walter Koenig, in the rebooted Star Trek films released in 2009 and 2013.
He reprised the role in a third film, Star Trek Beyond, which is due for release next month.
Justin Lin, who directed Star Trek Beyond, said: "Still in shock. Rest in peace, Anton. Your passion and enthusiasm will live on with everyone that had the pleasure of knowing you."
Zachary Quinto, who plays Spock in the franchise, wrote on Instagram: "Our dear friend. Our comrade. Our Anton.
"One of the most open and intellectually curious people I have ever had the pleasure to know. So enormously talented and generous of heart. Wise beyond his years. And gone before his time. All love and strength to his family at this impossible time of grief."
Fellow Star Trek actor John Cho, who plays Sulu, tweeted: "I loved Anton Yelchin so much. He was a true artist - curious, beautiful, courageous. He was a great pal and a great son. I'm in ruins."
Original Star Trek actor William Shatner also tweeted: "Sad news for such a talented young man."
The official Star Trek Twitter account marked his death with a simple, sombre image.
Jodie Foster, who directed Yelchin in 2011 film The Beaver, described him as "a rare and beautiful soul with his unstoppable passion for life".
Los Angeles police officer Jenny Hosier said Yelchin had just got out of the vehicle when it rolled back.
He had been on his way to meet friends for a rehearsal but when he did not arrive, the group came to his home and found him dead, she added.
LA police told CBS News the county coroner's office "concluded that Yelchin's death was an accident" caused by "blunt traumatic asphyxia".
Police also told the broadcaster the vehicle involved was a 2015 Jeep Grand Cherokee and it appeared Yelchin had not properly put the car in park mode.
CBS reported last month the government National Highway Traffic Safety Administration was investigating the model over its "e-shift" transmission, amid reports of crashes linked to possible confusion over whether the vehicle had been put into park.
Police investigators have not established any connection between the transmission in Yelchin's Jeep and the incident that took his life.
Felicity Jones, who starred opposite Yelchin in 2011 film Like Crazy, said: "I'm devastated. A world without Anton is a lesser place. He touched everyone he met with his honesty and humanity. We have lost a unique and profound soul."
Director Guillermo del Toro added he was "the sweetest, most humble, delightful, talented guy you'd ever meet".
2 Broke Girls star Kat Dennings tweeted: "Anton Yelchin was one of my best friends. Can't say anything that conveys what this feels like."
An only child, Yelchin was born to professional figure skaters in Leningrad (now St Petersburg) in Russia, who moved to the US when he was still a baby.
His first screen role was in US drama ER in 2000 and appeared in more than a dozen TV series and films before his break out role in 2006's Alpha Dog.
Among his other big screen credits were Charlie Bartlett (2007), Terminator Salvation (2009), Fright Night (2011), Odd Thomas (2013) and horror Green Room, released last year.
The Guardian's film critic Peter Bradshaw wrote an appreciation of the actor saying: "A hugely popular and well-liked actor of enormous talent and potential has been snuffed out. What a waste."
According to IMDB, he had four other film projects due for release - sci-fi Rememory, dramas We Don't Belong Here and Porto, and thriller Thoroughbred.