The lawsuit filed on Tuesday in a US District Court in Washington names CACI International and Titan Corporation, which had employees at the notorious Iraqi prison under a contract with the Pentagon.
The suit was filed under the Alien Tort Claims Act, which allows legal action to be brought in a US court concerning "violation(s) of the law of nations or a treaty of the United States".
The Iraqis "were unlawfully tortured by agents or employees of the defendants, who were under contract with the United States government to provide security and intelligence services" to US armed forces, according to the lawsuit.
The accusations covered a wide range of abusive conduct, including beatings, being deprived of food and water, being hanged by an injured arm from a rail, being photographed naked and being threatened with dogs and prolonged exposure to cold weather.
It maintains that the company and its employees "committed unlawful acts of torture ... for which there is no adequate or available remedy under Iraqi law".
The lawsuit seeks unspecified damages for the four Iraqis and the widow of the one who died after being tortured at the prison.
The Iraqis "were unlawfully tortured by agents or employees of the defendants, who were under contract with the United States government to provide security and intelligence services" to US armed forces.
Lawsuit filed in Washington
It marked the latest action since the scandal broke this year over photographs showing Iraqi prisoners humiliated and abused by US forces.
Last week, the US military revealed that there are a total of 94 cases of confirmed or alleged abuse of prisoners by US soldiers in Iraq and Afghanistan – a significantly higher number than Pentagon estimates.
The army's inspector-general made the revelations at a hastily called Senate hearing on 22 July.
On 18 July, investigative journalist Seymour Hersh accused the Bush administration of withholding tapes showing sodomy of Iraqi civilians.
"The boys were sodomised with the cameras rolling, and the worst part is the soundtrack of the boys shrieking. And this is your government at war," he told the American Civil Liberties Union (ACLU).
CACI International called the allegations against its employees in the suit "a malicious and farcical recitation of false statements and intentional distortions".
The plaintiffs include Ilham Nassir Ibrahim, widow of Akram Hanush Yaku, who - according to the lawsuit - died on 2 January after being tortured at the prison.
The other plaintiffs were named as Ali Shallal Abbas, Saddam Salih Abbud, Jilal Mahdi Hadud and Nasir Khalaf Abbas.
The Washington Post reported that in his deposition filed with the lawsuit, Abbud said he endured repeated beatings and that during one session, his hood was removed and he saw the prison commander, General Janis Karpinski.