Ventura, a rhythm and blues singer known as Cassie, also said Combs agreed to a $US20 million ($A31 million) settlement to end her November 2023 civil lawsuit against him over their relationship, with the deal coming 24 hours after she filed the case.
The disclosures came on the third day of Combs' criminal trial, where the 55-year-old rapper has pleaded not guilty to five felony counts of racketeering conspiracy, sex trafficking and transportation to engage in prostitution.
If convicted on all counts, Combs would face a minimum 15 years in prison and could face life behind bars.
Ventura, 38, fought back tears as she described Combs' allegedly raping her in her living room.
"I just remember crying and saying no but it was very fast," she said, as her voice trailed off.
Ventura said she sued Combs nine months after undergoing rehabilitation and trauma therapy prompted by "horrible flashbacks," and that she "didn't want to be alive" by the time she sought help.
Asked why she decided to testify against Combs, Ventura said she could no longer bear the emotional burden of years of his physical and emotional abuse.
"I can't carry this anymore," she said.
"I can't carry the shame, the guilt, the way he treated people like they were disposable. What's right is right, what's wrong is wrong. I came here to do the right thing."
Lawyers for Combs will begin their cross-examination of Ventura on Thursday, and it may last two days.
The lawyers have signalled they will ask Ventura about what they have called her own history of domestic violence.
Combs' trial could take up to two months.
Ventura is the prosecution's star witness against Combs, and wore a turtleneck dress and dark jacket for her second day of testimony.
Combs wore a cream-coloured sweater and white-collared shirt, with his mother and at least two children looking on.
Ventura testified that Combs beat her during drug-fuelled sex parties known as "Freak Offs," and that he threatened to release videos of her taking part and warned he would ruin her career.
Prosecutors showed jurors seven still photos of Ventura and various escorts that were taken from videos of "Freak Offs."
One juror swallowed hard upon seeing one image, and another jerked his head back.
The photos were not shown to spectators or the press. US District Judge Arun Subramanian, who oversees the trial, denied Reuters and other media limited access to the photos and videos.
Jurors were shown photos Ventura's mother once took of Ventura, where she sported bruises on the back and leg from having been kicked by Combs.
Ventura told jurors that fighting back against Combs sometimes slowed him down, but usually made the abuse worse.
"It would just make him more violent, make him stronger, make him want to push me harder," Ventura said.
"He said that it would ruin everything that I had worked for, that it would make me look like a slut. That I would be shamed. Nobody should do that to anyone."
Ventura acknowledged having herself initiated physical violence against Combs, but said she didn't see any injuries.
She said she was heavily dependent on opiates during her relationship with Combs.
By the time they broke up, she said she experienced episodes of post-traumatic stress disorder, sleepwalking and blackouts.
Ventura said she stopped doing drugs in 2022. She is pregnant with her third child.
Combs is being jailed in Brooklyn when not in court.
Also known during his career as Puff Daddy and P. Diddy, Combs founded Bad Boy Records and is credited with helping turn artists like Mary J. Blige, Faith Evans, Notorious B.I.G. and Usher into stars in the 1990s and 2000s.
Combs also faces dozens of civil lawsuits by women and men who accuse him of sexual abuse. He has denied wrongdoing, and said his relationships were consensual.
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