The man was caught on camera giggling with his head in his hands as he sat in the gallery in the courthouse in Fairfax, Virginia, watching the day’s testimony.
Several other spectators were spotted looking around at the man and also smirking at his reaction.
Seconds later, the man – apparently unable to regain composure – got up and walked out of the courtroom.
It is not clear what initially caused the man to erupt in a fit of laughter, but the incident on Wednesday came on the same day that emotional prerecorded testimony from Ms Heard’s former best friend Raquel Pennington was played to the court.
Ms Pennington broke down in tears as she testified that she was “scared” that “monster” Johnny Depp would “do something worse than he intended” to the Aquaman actress.
This isn’t the first time that the courtroom has descended into laughter during the multi-million-dollar trial.
Last month, several spectators were heard bursting out laughing when Mr Depp testified that he doesn’t watch his own movies and couldn’t recall what question he had been asked.
“I don’t watch them. I feel better not watching them. What was the question again?” Mr Depp said.
As laughter erupted in the room, the judge warned that people would be removed from the room if the was behaviour repeated.
“Order in the court, or I will have you removed,” Judge Azcarate said.
On another occasion, Mr Depp drew laughs when he joked that every hour is “happy hour” when he was asked about his issues with alcohol and drugs.
He also earned laughter at another point during his testimony when he predicted a hearsay objection from Ms Heard’s lawyers, telling the court “I’m learning”.
Mr Depp is suing his ex-wife for defamation over a 2018 op-ed she penned for The Washington Post where she described herself as a “a public figure representing domestic abuse”.
The Pirates of the Caribbean actor is not named in the article, which is titled “I spoke up against sexual violence – and faced our culture’s wrath. That has to change”.
However Mr Depp claims that it falsely implies that he is a domestic abuser – something that he strongly denies – and that it has left him struggling to land roles in Hollywood. He is suing for $50m.
Ms Heard is countersuing for $100m, accusing Mr Depp of orchestrating a “smear campaign” against her and describing his lawsuit as a continuation of “abuse and harassment”.