The unmanned underwater vehicle was taken around 50 nautical miles northwest of Subic Bay in the Philippines late on Thursday, according to the Pentagon, which called the capture unlawful and demanded its immediate return.
The incident comes amid escalating tensions between China and the United States, with President-elect Donald Trump repeatedly infuriating Beijing by questioning longstanding US policy on Taiwan, calling Beijing a currency manipulator and threatening Chinese imports with punitive tariffs.
China's defence ministry said it would give back the device "in an appropriate manner", without providing details of the handover.
"The hyping up from the American side is inappropriate and unhelpful to the swift resolution of the problem," the ministry said.
China said it "strongly opposed" US reconnaissance activities and had asked Washington to stop them.
"The Chinese side will take the necessary steps in response," the statement added.
The Pentagon said it had registered its objection to the probe's seizure.
"Through direct engagement with Chinese authorities, we have secured an understanding that the Chinese will return the UUV to the United States," spokesman Peter Cook said Saturday.
There are broader tensions in the South China Sea, where China has moved to fortify its claims to the region by building out tiny reefs and islets into much larger artificial islands.
Brunei, Malaysia, the Philippines and Vietnam have competing claims in the South China Sea, which is laced with the world's most heavily travelled international trade routes.
While the US takes no position on sovereignty claims in the South China Sea, it has repeatedly stressed all maritime claims must comply with international law.
Its military has conducted several "freedom of navigation" operations in which ships and planes have passed close to the sites Beijing claims.
