Vann Nath, a Cambodian artist who endured torture and near starvation as a Khmer Rouge political prisoner and who became a crucial witness to the regime’s atrocities through his later testimony against key officials, died Sept. 5 in Phnom Penh, Cambodia.

The Associated Press reported his death but did not include a specific cause. He had suffered from chronic kidney disease and had fallen into a coma in late August, his daughter Vann Chan Simen told the news service.

Mr. Vann Nath, whose age was variously reported as either 65 or 66, was spared from execution when a guard discovered his artistic talents and forced him to paint portraits of the Khmer Rouge’s despotic ruler, Pol Pot.

Mr. Vann Nath emerged in 1979 from his year-long imprisonment in Phnom Penh and became one of Cambodia’s most revered artists. His paintings of the torture and killings he witnessed under Pol Pot served as testimonials to Cambodia’s darkest era and earned Mr. Vann Nath the nickname Goya of genocide.

It is estimated that nearly 2 million Cambodians perished between 1975 and 1979 under the Khmer Rouge.

In the decades since the Vietnamese army ousted the regime, thousands of skeletal remains of Pol Pot’s victims have been unearthed from the so-called Killing Fields outside the capital city.

Mr. Vann Nath was among 15,000 people arrested as “enemies of the state” and sent to S-21, a French lycee turned secret prison, known as Tuol Sleng.

Of Tuol Sleng’s prisoners, Mr. Vann Nath became one of perhaps a dozen known to have survived. The rest died of starvation or disease or were executed in the Killing Fields. Many succumbed during interrogations overseen by guards, whose preferred tool of bodily mutilation was a pair of pliers.

Mr. Vann Nath spent the first month of his incarceration in a cell packed with 50 other men, shackled in a prone position. He was beaten and subjected to electrical shocks. He ate a few spoonfuls of gruel a day.

The guards sometimes took hours to retrieve the corpses of the prisoners who died beside him. Mr. Vann Nath later recalled that he felt so hungry that he contemplated cannibalism.

“We didn’t care because we were like animals,” he said.

He recounted how painting became his savior in a 1998 memoir, “A Cambodian Prison Portrait: One Year in the Khmer Rouge’s S-21,” and in the 2003 documentary “S21: The Khmer Rouge Death Machine.”

The prison commander, Kaing Guek Eav, known as Duch, discovered that Mr. Vann Nath had been trained as an artist.

The commander gave Mr. Vann Nath a photograph depicting a man whose face the painter did not recognize. He later learned it was Pol Pot.

Duch ordered Mr. Vann Nath to create a noble portrait of the regime’s leader. He painted from dawn until past midnight for several days.

“Every brush stroke, you were just hoping that they would like it and would let you live,” Mr. Vann Nath told the New York Times in 2002.

His reproduction was deemed suitable by Duch. Mr. Vann Nath was given extra rations and allowed to continue painting in a small workshop away from the other prisoners. He heard their screams echo outside his door.

“If I didn’t know how to paint,” he told Newsweek magazine in 2001, “I would be just another skull in the Killing Fields.”

When Vietnam invaded Cambodia, the Tuol Sleng jailers fled, and Mr. Vann Nath escaped. Today, his paintings hang on the walls of the old prison, which is now a museum. Many of its visitors are Cambodians born in the years since Pol Pot’s regime ended.

“There is a danger the young generation will forget,” Mr. Vann Nath told Newsweek. “That’s why I painted these: so history wouldn’t repeat itself.”

Mr. Vann Nath was born to a farming family that lived in Cambodia’s northwestern Battambang province.

He was a Buddhist monk for several years before studying to become an artist. He made money painting billboards and movie posters. When Pol Pot came to power, he forced millions of people into slave labor. Mr. Vann Nath was working in a rice field in 1978 when he was arrested. He never learned why he was targeted by the regime.

After his imprisonment, he owned a restaurant and sold his paintings. A complete list of his survivors could not be confirmed.

Two years ago, Mr. Vann Nath testified in a tribunal against Duch, the prison commander, who was charged for crimes against humanity. In 2010, the tribunal sentenced Duch to 35 years in prison.