The US ambassador to Bangladesh condemned the killing of Xulhaz Mannan, who also worked at the US embassy.
Since February last year suspected militants have killed a number of secular and atheist writers.
Mannan was murdered two days after a university teacher was hacked to death by suspected militants.
ISIS has claimed responsibility for the murder but the Bangladeshi government have claimed that there is no ISIS presence in the country.
“I am devastated by the brutal murder of Xulhaz Mannan and another young Bangladeshi,” said US Ambassador Marcia Bernicat.
“We abhor this senseless act of violence and urge the government of Bangladesh in the strongest terms to apprehend the criminals behind these murders,” she added.
Homosexuality is technically illegal in Bangladesh and remains a highly sensitive issue in society.
Mannan was openly gay and believed that if more gay Bangladeshis came out then the country would have to accept them, a BBC source said.
He was also behind the annual “Rainbow Rally”, held on the “Bengali New Year”, 14 April, since 2014. This year’s rally was banned by police as part of widespread security measures.