Private contractors were called in to skim oil from the surface of the water and place thousands of feet of boom to contain the floating slick.
A stretch of the busy river was closed to commercial and recreational traffic while the spill was being cleaned up. The Coast Guard asked residents to call the U.S. Fish and Wildlife Service if they see animals affected by the spill.
"I would classify any spill in the water as significant," said Coast Guard Petty Officer John Edwards. "We're working very quickly and diligently to expedite the cleanup."
Two tugboats were guiding the ship to a pier Friday night when a tugboat skipper noticed the spill, said Coast Guard Capt. Jonathan Sarubbi, officer in charge of the Port of Philadelphia. The ship listed 8 degrees to the left at about the same time, he said.
The crew notified the Coast Guard and began transferring oil from the leaking tank to another tank on board. The leak was stopped within an hour.
The cause of the spill was still under investigation, Sarubbi said.
The tanker, the Athos I, registered in the Mediterranean island nation of Cyprus, was carrying 325,000 barrels of oil from Venezuela, said Jim Lawrence, a spokesman for the vessel's owner, Greek shipping company Tsakos Shipping and Trading SA.
The nearly 750-foot-long ship (230-meter-long-ship), built in 1983, was last out of the water for maintenance in April, Lawrence said. He said it had never before spilled oil.
It was the worst spill on the Delaware River since 1995, when strong wind pushed a tanker away from a refinery dock in West Deptford, New York, snapping a fuel line that spilled 40,000 gallons (150,000 liters).
About one million barrels of oil come through the Port of Philadelphia each day.