Offering his party's unconditional support to the bill, Gandhi in the letter said the time has come for a change, and for women to take their rightful place in state legislatures and Parliament.
"I write to request (for) your support to ensure the passage of the Women's Reservation Bill in the coming monsoon session," he said.
Gandhi said that Modi had spoken about his passion for empowering women in many of his rallies in the past.
"Mr Prime Minister, in many of your public rallies you have spoken about your passion for empowering women and involving them more meaningfully in public life.
"What better way to demonstrate your commitment to the cause of women, than by offering your unconditional support to the passage of the Women's Reservation Bill? And what better time, than the upcoming session of Parliament? Any further delay will make it impossible to implement before the next general elections," Gandhi wrote to Modi.
The monsoon session is slated to begin on July 18 and will continue till August 10.
He said the Women's Reservation Bill, which was passed by the Rajya Sabha on March 9, 2010, has been stalled on one pretext or the other in the Lok Sabha, for over eight years now.
Since then, while the Congress has been unwavering in its commitment to the bill, the BJP appears to have had second thoughts, even though this was one of its key promises in its 2014 manifesto, Gandhi said.
"On the issue of empowering our women, let us stand together, rise above party politics and send India a message that we believe the time for change has come. Women must take their rightful place in our state legislatures and in Parliament, where they are at present abysmally represented," he wrote to the prime minister.
"For the skeptics in your party who oppose the passage of this transformational (proposed) legislation, I would appreciate you pointing out to them that our nation's experience with governance at the Panchayat and Municipal levels, has clearly shown that women in leadership positions are more likely, than their male counterparts, to take decisions that further the cause of an inclusive and just society; which is why this bill has the potential to transform governance in India," the Congress president said.
(With inputs from agencies)