The Pentagon and China’s Defense Ministry said an American underwater survey drone that a Chinese naval vessel seized in the South China Sea would be returned to the U.S., ending a brief standoff that comes as China shows resistance to U.S. maritime surveillance in the region.

Pentagon Press Secretary Peter Cook said on Saturday that the U.S. had “secured an understanding” that China would return the underwater survey drone, after engaging directly with Chinese authorities. He didn’t say when or how the device, which a Chinese...