The plane had arrived from Glasgow and was full of passengers heading to Alaska when authorities were alerted. All 157 passengers and six crew members on board and their luggage were removed.
The passengers were taken inside to one of the airport's terminals. Across the tarmac, baggage handlers unloaded the Boeing 757.
Officials said the incident was referred to the FBI, but the FBI didn't immediately comment on the nature of the threat or what steps it was taking in the investigation.
Philadelphia International Airport, which handled 31 million passengers last year, is the only major airport serving the Philadelphia area, one of the largest metropolitan areas in the United States.
The plane was cleared by authorities shortly before 6 p.m. Pacific Time.
The Associated Press contributed to this report.