Muslims, Christians and men of other minority faiths could face up to a decade in prison for marrying Buddhist women under a law being considered by parliament, according to the New York-based watchdog.
The country formerly known as Burma has been shaken by religious strife as it emerges from decades of oppressive military rule, with at least 250 people killed in Buddhist-Muslim clashes since 2012.
President Thein Sein, a former general turned political reformer, last month asked parliament to consider proposed intermarriage restrictions, after a campaign spearheaded by a hardline monk.
"It is shocking that Burma is considering enshrining blatant discrimination at the heart of Burmese family law," said HRW Asia director Brad Adams.
"This law would strip away from women their right to freely decide whom to marry, and would mark a major reversal for religious freedom and women's rights in Burma," he said in a statement.
In a letter to lawmakers, Thein Sein said the proposed legislation was to give "protection" to Buddhists marrying people of other religions.
Full details of the draft law have not been published.
But HRW said a version it had seen would mean that Buddhist women would only be allowed to marry Buddhist men.
The watchdog said it would also require men to seek permission in writing from a Buddhist bride's parents before marriage, "seriously jeopardising women's autonomous decision making".
Myanmar is thought to be around 89 percent Buddhist, with Christians and Muslims each making up around four percent -- although experts believe the true proportion of religious minorities could be higher.
Sectarian bloodshed -- mostly targeting Muslims -- has laid bare deep divides that were largely suppressed under nearly half a century of military rule, which ended in 2011.
Radical monks -- once at the forefront of the pro-democracy movement -- have led a campaign to shun shops owned by Muslims and only to visit stores run by Buddhists. Some were also involved in the religious unrest.
"In ethnically and culturally diverse Burma, government leaders are playing with fire by even considering proposals that would further divide the country by restricting marriage on religious lines," Adams said.
Opposition leader Aung San Suu Kyi has slammed the interfaith marriage proposals as "a violation of women's rights and human rights".
Myanmar has already faced criticism over a "two-child policy" in some areas targeting Rohingya Muslims, considered by the United Nations to be among the world's most persecuted minorities.