The unarmed target drone was deployed from the guided missile destroyer USS Chafee in war games off the coast of Guam in September last year, the US embassy in Manila said in a statement.
"It appears the ocean currents brought the drone to where it washed ashore last week off Masbate island," it said.
Fishermen found the drone drifting just off the coast of the island in central Philippines, about 2,500 kilometres (1,553 miles) from Guam, at the weekend.
President of the Philipine, Benigno Aquino, had previously said US drones are allowed to conduct reconnaissance missions in the Philippines as part of efforts to contain a range of security threats.
The Philippine military has said drones are used during annual joint military exercises with the US in its waters, while some 600 US forces have been in the southern Philippines since 2002 to help train local troops to deal with Islamic militants.
Nevertheless the discovery of the drone stirred controversy in the Philippines, prompting local authorities to insist the machine was not being used for spying or to fire weapons at targets.
"It is not a spy plane. It is unarmed. It is used by the US Navy in its training," Philippine military spokesman Colonel Arnulfo Burgos told AFP.
The Philippine department of foreign affairs released a similar statement, although qualifying it by emphasising its comments were based on assurances from the US government.
"At this point, we have been assured by the US Embassy that the reported aerial vehicle is by design and purpose solely used for target practice and not armed or used for surveillance," the statement said.
Renato Reyes, secretary-general of leftist political group New Patriotic Alliance, alleged the drone was proof the United States -- a former colonial ruler of the Philippines -- was violating Filipino sovereignty.
"No sovereign nation would allow a foreign power unhampered use of domestic airspace," Reyes said in a statement.