Niraz Saied, who himself hailed from the Palestinian camp, was arrested by security forces in October 2015.
His longtime partner, Lamis Alkhateeb, wrote on Facebook on Monday that Saied had died while in detention. He was believed to be 27 years old.
"There's nothing harder than writing these words, but Niraz doesn't die in silence," wrote Alkhateeb, who lives in Germany.
"They killed my darling, my husband, my Niraz -- they killed you, my soul. Niraz died in the Syrian regime's prisons," she wrote.
It was not clear how Alkhateeb had learned of Saied's death, and she did not immediately respond to AFP's request for additional comment.
Their relationship had formed part of the 2014 film "Letters from Yarmuk," which featured clips filmed by Saied of daily life in the battered, besieged camp.
That same year, Saied won a photography competition run by the United Nations' Palestinian agency (UNRWA) with a snapshot titled "The Three Angels."
It depicted the downtrodden faces of three brothers waiting to be evacuated from the camp for medical treatment.
"You can't find a complete family in the refugee camp," Saied said after winning the award.
"I used to feel that in every portrait of a Palestinian family you could see the shadow of a person missing, and that is why my photos are dimly lit. But there is always hope."
Yarmuk was once a thriving southern district of Syria's capital home to more than 160,000 Palestinian refugees as well as Syrians.
Syria's government imposed a crippling siege on it in 2012 and activists inside -- including Saied -- documented the dire humanitarian situation with photographs of gaunt families waiting for aid.
Tens of thousands of people are believed to have been forcibly disappeared since Syria's conflict broke out in 2011, the vast majority by government forces.
Rights groups have accused the regime of large-scale torture and extrajudicial killing in its prisons.
Families of detainees often hear nothing after the arrest, but in recent months some are discovering their detained relatives have been officially registered as deceased.
The Syrian Observatory for Human Rights, a Britain-based monitor, said that within less than a month, some 28 families were either informed their detained relative was dead or told to come retrieve the body.
Hundreds more discovered their relative was recorded as "deceased" by government agencies while filing other kinds of paperwork.
Saied's childhood friend Ahmad Abbasi described him as "the finest person I knew".
"In the early days of his detention, we heard that he was still alive. Then we didn't know anything."
