Transformers: Dark of the Moon star LaBeouf recounted a meeting with Bieber at the MTV Movie Awards during an appearance on ABC's Jimmy Kimmel Live on Thursday night.
"He rolls up with a huge squad of dudes - he has to," said LaBeouf. "He has this presence like Buddha or a young prince.
"I went up to say hi and the security is like, 'Wow, wow' and ready to take me down. Bieber does this finger wave like, 'He's OK'. It was very cool to me, super cool! Bieber is the bomb. He's like a 16-year-old Frank Sinatra. He has so much presence."
LaBeouf recently admitted that he was unsure if he would want to achieve the global fame of stars like Bieber or Robert Pattinson, telling Italian magazine Un'Intervista that he prefers a more low-key existence.
"Popularity is like a drug," he told the publication. "I would not want the worldwide popularity of Robert Pattinson or Justin Bieber. Even if you don't reach those levels, it's hard not to feel at least a little almighty."
LaBeouf recently met Bieber's girlfriend Selena Gomez, who posted a video of their encounter on her Twitter feed alongside the message: "Best. Day. Ever."
In the video, Gomez can be seen entering a dressing room backstage at a television studio thinking that she is about to meet some fans, only to see LaBeouf waiting inside, prompting her to flee back into the corridor.
LaBeouf can be seen in Transformers: Dark of the Moon, which is in cinemas now.
> Read our full Transformers headlines
Watch more of Shia LaBeouf's interview on Jimmy Kimmel Live below:
Watch our interview with Michael Bay and Rosie Huntington-Whiteley below:
Watch the trailer for Transformers: Dark of the Moon below: