In her letter, she said sexual violence against women and children in the country had reached critical levels.
Ms Maliwal highlighted the rape of an eight-month-old girl in Delhi, the rape and murder of an eight-year-old girl in Kathua in Jammu and Kashmir and the Unnao case.
She appealed to the prime minister to "think about these girls" during his fast today and work towards securing justice for them.
The women's commission chief wrote that taking inspiration from the prime minister's fast, she would go on an indefinite hunger strike from tomorrow until the prime minister comes up with a concrete plan to check the incidents of sexual assault against women in the country.
Follow NDTV for latest election news and live coverage of assembly elections 2019 in Maharashtra and Haryana.
Subscribe to our YouTube channel, like us on Facebook or follow us on Twitter and Instagram for latest news and live news updates.