In a speech, he recalled confessing to a priest how he reached into the maid's underwear as she slept.
Women's rights group Gabriela said Mr Duterte was "unworthy of his position and should resign".
The Philippines president has sparked outrage with a number of provocative comments but remains popular.
In his latest remarks, he described entering the room of his maid while she was sleeping.
"I lifted the blanket... I tried to touch what was inside the panty," he said. "I was touching. She woke up so I left the room."
It is common for well-off families in the Philippines to employ maids and many women also work as domestic helpers across other Asian countries and the Middle East.
His spokesman since said Mr Duterte had merely "made up" and "added and spliced" the story.
Remarks 'encourage rape culture' Rights groups in the Philippines have strongly condemned the president's comments. Gabriela said the comments were tantamount to admitting rape.
"Rape does not happen only through penile insertion," Joms Salvador, secretary general of Gabriela said. "If it is a finger or an object it is considered rape."
The Coalition Against Trafficking in Women-Asia Pacific warned that the president's comments put domestic workers at risk.
"Flaunting abusive practices encourages the rape culture and in this case, sexual abuse of domestic workers," said Jean Enriquez, the organisation's executive director.
Mr Duterte is especially popular among the Philippines' poorer population, where many women work as domestic helpers.
Latest controversy The Philippines president has drawn fire for previous actions and comments about women.
Earlier this year, he sparked criticism for kissing an overseas Filipina worker on the lips during a live event.
He also told Filipino soldiers they should shoot female communist rebels in the vagina.
In April 2016, he spoke to an election campaign rally about the 1989 murder and rape of a female Australian missionary in Davao, where he was mayor at the time.
"I was angry because she was raped," he said. "That's one thing. But she was so beautiful, the mayor should have been first, what a waste." His office later apologised. - BBC