Census estimates show Philadelphia's population as of July 1 stood at 1,553,165.
Since 2006, the city has added more than 64,000 people but the new census numbers suggest that the population rise has slowed.
The city's population increased by 4,518 in 2013 - the seventh consecutive year of growth. According to census estimates, however, that increase is less than the 10,150-person increase in 2012 and the 12,491-person increase in 2011.
Philadelphia's growth last year primarily stemmed from births and foreign immigrants.
After peaking at more than 2 million people in 1950, the city's population began a 50-year decline that paralleled the loss of its manufacturing base.
The new estimates are projections based on birth and death records, as well as migration data based on recent American Community Survey results, federal tax returns, group-housing data such as prisons and dorms, and other data sources.
It is too soon to know if a downward trend may be taking hold. But the data raise questions about whether problems in city schools and the rising cost of housing are affecting residents' wherewithal to remain in Philadelphia, said Temple University researcher and demographer David Elesh.
"It is one year, and one year does not qualify as a trend, but it is worth looking at," he said.