The Pirates of the Caribbean star (58) is suing Heard in Virginia for $50 million, saying she defamed him when she said she was a victim of domestic abuse. Heard (36) has countersued for $100 million, arguing that Depp smeared her when his lawyer said her claims were a "hoax."
Depp's attorneys on Wednesday called Moss as a rebuttal witness after Heard had testified about a time she feared Depp would push her sister down a staircase.
Heard said she recalled "rumours" of Depp pushing Moss down a flight of stairs and she slapped Depp to stop him from harming her sister.
Moss (48), who dated Depp from 1994 to 1998, testified on Wednesday that she slipped on a staircase and injured her back during a vacation the couple took in Jamaica.
Depp rushed to help her, carried her to her room and summoned medical services, she said.
"He never pushed me, kicked me or threw me down any stairs," said Moss, who appeared via video from England.
Closing arguments in the case, which is being heard in Virginia, are scheduled for Friday.
Depp and his attorneys on Wednesday worked to rebut Heard's testimony that he physically and sexually abused her multiple times before and during their marriage, which ended in 2017, and said that it was she who turned violent.
He offered a string of adjectives to describe his reaction to Heard's accounts during the trial, now in its sixth and final week.
"Horrible, ridiculous, humiliating, ludicrous, painful, savage," Depp said, adding "unimaginably brutal, cruel and false. All false."
"No human being is perfect. But I have never in my life committed sexual battery, physical abuse."
Depp told jurors it was Heard who turned violent in their relationship. Closing arguments in the trial being held in Virginia are scheduled for Friday.
"No matter what happens," Depp said, "I did get here and I did tell the truth and I have spoken up for what I’ve been carrying on my back, reluctantly, for six years."
In cross-examination, Heard's attorneys confronted Depp with text messages retrieved from his cell phone. One message to an assistant described a woman's genitals as being "RIGHTFULLY MINE."
"I NEED. I WANT. I TAKE," the message said.
Depp denied he wrote the texts and said his phone might have been used by someone else or the message was altered.
Heard, best known for her role in Aquaman, met Depp in 2011 while filming The Rum Diary and the pair wed in February 2015. Their divorce was finalised about two years later.
At the centre of the legal case is a December 2018 opinion piece by Heard in the Washington Post. The article never mentioned Depp by name, but his lawyer told jurors it was clear that Heard was referring to him.
Depp, once among Hollywood's biggest stars, said Heard's allegations cost him "everything." A new Pirates movie was put on hold, and Depp was replaced in the Fantastic Beasts film franchise, a Harry Potter spinoff.
Heard's attorneys have argued that she told the truth and her opinion was protected free speech under the US Constitution's First Amendment.
Depp lost a libel case less than two years ago against British tabloid the Sun that labelled him a "wife beater."
A London High Court judge ruled that he had repeatedly assaulted Heard.
Depp's lawyers filed the US case in Fairfax County, Virginia, because the Washington Post is printed there. The newspaper is not a defendant.
