Authorities said at least 12 women are in the prison's population of 7,000.
Syrian rights activists reported that female inmates at Adra are verbally harassed by male guards, restricted to their cells longer than men and must get Political Security approval for family visits while male prisoners don't, the rights organization said Friday in a release.
"We don't know why Syria is keeping these women in Adra prison, but we do know that their situation is precarious and they are being treated worse than their male counterparts," said Sarah Leah Whitson, Human Rights Watch's Middle East director.
The United Nations-issued Standard Minimum Rules for the Treatment of Prisoners states that men and women "so far as possible" will be held in separate institutions, and it bars male corrections officers from entering female sections of a prison unless accompanied by a woman officer.
Syrian officials said women officers guard the female inmates, HRW said.
Syria has prisons for women, including a main facility in Douma, in the suburbs of Damascus, Human Rights Watch said.
HRW said it couldn't determine why the women were housed in the men's facility.