The star sued his ex-wife for $50 million (£39 million) after she referred to herself as "a public figure representing domestic abuse" in a Washington Post article in 2018. He claimed he had been defamed in the piece, even though Heard didn't name him, stating it suggested the actress was a domestic abuse victim and he "perpetrated domestic violence against her."
"Aquaman" actress Heard has already launched a $100 million (£78 million) counterclaim against Depp this summer (20) - but in a new tweet, "The Greatest" singer Sia insists Depp is the "victim" in the spat.
"Just showing my public support for Johnny Depp," she penned. "I mean, I'd love him to get clean and stop with the jewelry, but he is clearly the victim after hearing those tapes."
The tapes the star refers to appear to be audio recordings that leaked online in February, in which Heard admits to "hitting" her ex in the past as the pair discuss their marital problems and the physical violence in their marriage.
Depp and Heard split in 2016 after Heard accused Depp of being abusive throughout their 15-month marriage. The "Pirates of the Caribbean" actor has vehemently denied the claims.
The former couple are currently embroiled in a court battle as Johnny Depp launched a defamation lawsuit against Amber Heard. They also met in court in another libel trial that pitted Depp against British tabloid.