Eric Ohena Lembembe was found dead in his bed Monday night by friends who had been unable to contact him for two days, the rights group said in a statement.
Lembembe's neck and face appeared to have been broken and his face, hands and feet burned with an iron, a friend said.
Lembembe was a prominent advocate for the rights of gay, lesbian, bisexual, transgender and intersex people. The organization he headed, the Cameroonian Foundation for AIDS, documented the arrests and incidents of violence and blackmail against LGBTI individuals in Cameroon.
Neela Ghoshal, senior LGBT rights researcher for HRW, charged "the Cameroonian authorities' utter failure to stem homophobic violence sends the message that the attacks can be carried out with impunity."
She called on President Paul Biya to "break his silence on the wave of homophobic violence in Cameroon and publicly condemn this brutal attack."
Last month, the Yaounde office of attorney Michel Togue, who represents clients charged with same-sex conduct, was burglarized and his laptop and files stolen. He and another attorney, Alice Nkom, who represents LGBTI clients, have received death threats, including threats against their children.