The Rev. Federico Lombardi, a Vatican spokesman, said Friday that Francis had expressed "his willingness to participate" in the triennial World Meeting, scheduled for Sept. 22 to 27 next year.
The international gathering is expected to draw about 300,000 visitors a day for its conferences promoting traditional values and Catholic teachings. But a visit by the pope to say Mass - traditionally scheduled for the final day - would likely draw more than a million people.
"The pope will be with us the Friday, Saturday, and Sunday of that week," the Catholic News Service reported Chaput as saying during the opening Mass of the Tekakwitha Conference in Fargo. He also invited those in attendance to come to Philadelphia for the gathering.
The archdiocese said Friday it expected that "any official confirmation will come approximately six months prior to the event."
Lombardi said Francis was "considering invitations" from New York, Washington, the United Nations, and Mexico.
A three-day visit would be a departure from tradition. Francis' immediate predecessors - Benedict XVI and John Paul II - usually appeared on the final day of the popular international gatherings to say Mass.
In March, Chaput led a delegation of city and state civic leaders, including Mayor Nutter and Gov. Corbett, to meet with Francis in Rome and formally invite him to the city for the World Meeting.
Those officials had only brief conversations with Francis during his regularly scheduled public audience in St. Peter's Square, but came away voicing confidence that Francis would come.
Officials of the Pontifical Council for the Family, the Vatican office that sponsors the World Meetings, also hinted strongly in March that Francis was strongly inclined to attend, saying that only ill health would likely deter a visit.
Nevertheless, Chaput's remarks appeared to take the archdiocese by surprise. It did not issue a response until late in the morning.
"Archbishop Chaput has frequently shared his confidence in Pope Francis' attendance at the World Meeting, and his personal conversations with the Holy Father are the foundation for that confidence," it said.
"We are further heartened and excited by the comments of the Vatican Press Office regarding Pope Francis' 'willingness to participate in the World Meeting of Families.' While Archbishop Chaput's comments do not serve as official confirmation, they do serve to bolster our sincere hope that Philadelphia will welcome Pope Francis next September."
The ability of the city's tourism infrastructure to accommodate the World Meeting of Families - with or without the pope - is enhanced because True Value, a cooperative of 5,000 hardware stores, canceled its 2015 convention at the Convention Center.
Organizers of the True Value show complained about the center's workforce during the group's September 2011 convention. Its 2015 convention had been booked for Sept. 18 to 20 - just days before with the World Family meeting. The World Meeting needed more than a week to set up at the center, which would have presented a conflict.
The Convention and Visitors Bureau estimates that the World Meeting of Families would generate $15.5 million in economic impact for the city, including money paid to book 26,050 nights in hotels. With the pope visiting, the economic impact increases to $231.9 million, including 50,000 nights in hotels.
Inquirer staff writer Jane M. Von Bergen contributed to this article.