If current rates continue, Philadelphia is set to drop even further within the next five years, overtaken by Miami and Atlanta.
Numbers show Philadelphia is still growing. But its drop in rank isn't arbitrary: It is symptomatic of a region that continues to struggle with high taxes, a city school system in chaos, and industries that aren't hiring at the rates they did in the region's heydays.
Economies elsewhere "are growing more quickly than ours," said David Elesh, an urban sociology professor at Temple University. "Between that and their climate in some cases, they can attract the residents and the retirees."
Nevertheless, in 2015 metro Philadelphia grew for a ninth consecutive year - even if, compared with sunnier regions exploding with growth, Philadelphia is merely inching along.
In total, the population of the metro area - which includes parts of New Jersey, Delaware, and Maryland - grew by nearly 105,000 residents between 2010 and 2015, bringing it to just above six million.
Yet that growth yielded only a 1.6 percent population increase for the metro area. Nationally, the population grew 3.9 percent between 2010 and 2015.
"We're encouraged by the continued growth, but we know there is still much work to be done to increase our competitiveness with other metro areas," said Lauren Hitt, spokeswoman for Mayor Kenney.
While a drop from the sixth most populous region in 2014 to seventh in 2015 may seem incremental, experts say there's cause for concern: The region's growth was largely fueled by births offsetting others' departures.
Although residents have moved out, nearly 50,000 immigrants came to the city of Philadelphia in the same time period - and their numbers, experts said, are what counterbalance other losses, as in other Eastern cities.
"What's keeping us from declining, for the most part, is immigration from abroad," said James W. Hughes, dean of the Edward J. Bloustein School of Planning and Public Policy at Rutgers University.
Thursday's rankings come as the city of Philadelphia has been experiencing something of a renaissance, becoming what locals, developers, and officials have dubbed as a trendy destination for millennials.
The experts say they see evidence of a cultural revitalization in Philadelphia. Neighborhoods that were once depressed have undergone gentrification, attracting new housing and new residents. Erstwhile working-class neighborhoods in Fishtown and South Philadelphia have sprouted wine bars and coffee shops, yoga studios and trendy restaurants.
"We do pretty well for a Northeastern city in terms of holding our own," said Mary Bell, manager of demographic and economic analysis for the Delaware Valley Regional Planning Commission.
Millennials, however, are not enough to offset other age groups that are leaving - and a big question remains: Will the 20-somethings stay once they start having children?
"Young people are staying in Philadelphia," Elesh said. "It's people who have families, people who have children" who are leaving - wary of a public school system in flux.
And they are not necessarily going to the neighboring suburbs.
South Jersey has been particularly hard-hit by the recession, the experts said. Some of the nation's highest property taxes, income taxes, and an absence of qualities that attract millennials - including strong public transit and walkable downtowns - have also contributed to the slowdown, Hughes said.
While Montgomery and Chester Counties were growing - Chester leading the region with a 3.2 percent population increase between 2010 and 2015 - Delaware and Bucks Counties barely moved the needle.
"The metro area is growing slowly and we're part of that," said David Zipf, a community planner at the Bucks County Planning Commission.
In cities such as Dallas, Houston, Miami, and Atlanta, faster growth is no surprise, experts say, as they have been buoyed by warmer climates and fresh development. But Washington, just three hours to the south, is also gaining domestic residents while Philadelphia is losing them.
Largely thanks to jobs tied to the federal government, the D.C. economy has remained strong, while the collapse of Philadelphia's industrial sector left this region reeling. Plus, the district also has cultivated a high appeal among city-living millennials.
"As long as there's the U.S. government . . . there will be opportunities in the Washington area," said Blair Ruble, vice president of the Wilson Center, a Washington-based research organization.
As long as Philadelphia is growing, that's good news, experts said - but the region could always do more to compete.
"I'm not surprised they surpassed us," Bell said. "They were right on our tails as of last year and still gaining."
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