It looks like the Philadelphia 76ers won’t be leaving South Philly after all.

After the city officially backed a deal last month for a new arena in the heart of downtown, a proposal that had long drawn vociferous opposition, the basketball team’s owners announced on Monday that they now planned to build a new arena in the same sports and entertainment district where they have played for decades.

At a news conference at City Hall, the team’s owners joined with executives from the entertainment conglomerate Comcast to announce that the Wells Fargo Center, which the team has called home for years, would be replaced with a “world class arena” in the sports complex area. Wells Fargo — which is owned by Comcast and which the Sixers share with the Flyers of the National Hockey League — is a short walk from the stadiums for the Eagles, of the National Football League, and the Phillies, of Major League Baseball.

Mayor Cherelle Parker described the news as “a win-win-win-win,” but the announcement was a stunning turnaround — a fact that she acknowledged. “This is a curveball that none of us saw coming.”

For months and even as recently as last week, the mayor had been an ardent champion of a new arena in Center City. It is a part of downtown that has been struggling, and the structure was seen a big political win for the mayor in her first year in office.

In outlining the new plan, she highlighted elements beyond the building of a new arena, including efforts to attract a W.N.B.A. team to the city and a commitment by the new Sixers-Comcast partnership to invest in the revitalization of Market Street East, the Center City neighborhood where the venue was supposed to be built.

“I don’t have the option of wallowing in a 180,” Mayor Parker said. “This is a celebration for the city.”