Sindh's provincial assembly voted to criminalise those who use force to make people change their religion.
The bill means perpetrators can face five years in prison whilst facilitators could be handed a three year sentence.
According to charities at least 1,000 girls, most of them Christians, are forcibly converted to Islam in Pakistan every year.
Physical, emotional or psychological violence is often used to make another person adopt another religion.
Under the new law anyone converting will be monitored for 21 days "to ensure they are converting for religious purposes and by their own free will, not out of fashion or under force."
Nasir Saeed, director of CLAAS-UK, welcomed the new law: "Unequivocally, it is a great step taken by the Sindh government to protect the religious minorities.
"Though it will not stop the ongoing trend in society, it will help to reduce such crimes and it will also build confidence and a sense of security among the religious minorities.
"The police do not normally take action in such cases, fearing the reaction and pressure from the Muslim community, but now they will have to take action."
He urged the Punjab government to consider a similar law given that it is home to most of the Christian population.
