The decision was taken by Chief Minister Akhilesh Yadav owing to the theme of the film which portrays the battle of a woman against sexual offenders and crimes against women, a state government spokesman said.
The spokesman said the film depicted serious social evils like crimes and sexual offences against women and gave a positive message to society.
The film shows a woman's fight against such evils and it was therefore decided by the government to make it tax-free, he added.
Directed by Pradeep Sarkar, "Mardaani" stars Rani as police officer Shivani Shivaji Roy, who fights the girl trafficking mafia.