Economists had estimated that the index fell to 12.0 from 15.2 in June, according to Bloomberg.
Earlier this week, Bank of America Merrill Lynch highlighted research showing that this index is the best indicator of economic growth.
And in their preview of the data, the bank's economists wrote: "The surprise jump last month was likely a one-off, as the index had been averaging only 6.1 to start the year. That said, the US economy seems to be broadly strengthening, which should provide some lift to the Philly Fed district's manufacturing sector. The strong dollar remains a headwind, however, and the recent tumble in oil and commodity prices may yield heightened uncertainty. Developments in Greece and China could also provide some downside pressure."