The 141-page report released Tuesday by Human Rights Watch said politically motivated sexual violence by police and the military continues even though armed conflict with the separatist group ended in May 2009.
The report details 75 cases of rape and sexual abuse of men and women that occurred from 2006 to 2012 in both official and secret Sri Lankan detention centers.
"The Sri Lankan security forces have committed untold numbers of rapes of Tamil men and women in custody," said Brad Adams, Asia director for Human Rights Watch.
Men and women reported being raped on multiple days, often by several people, with the army, police and pro-government paramilitary groups frequently participating.
Victims also described being beaten, hung by their arms, partially asphyxiated and burned with cigarettes.
Human Rights Watch said it wants the United Nations High Commissioner for Human Rights Council to conduct an independent international investigation of the situation.