China said it "strongly opposed" US reconnaissance activities and asked Washington to stop.
The Chinese navy handed over the drone near the location it had been seized, the Pentagon said, repeating US condemnation of Beijing's actions in what it says are international waters.
"This incident was inconsistent with both international law and standards of professionalism for conduct between navies at sea," Pentagon Press Secretary Peter Cook said in a statement late Monday.
"The US has addressed those facts with the Chinese through the appropriate diplomatic and military channels, and have called on Chinese authorities to comply with their obligations under international law and to refrain from further efforts to impede lawful US activities."
A Chinese naval vessel seized the probe around 50 nautical miles northwest of Subic Bay in the Philippines last week in an incident that heightened already tense relations between the world's two largest economies.
The Pentagon statement said the US Navy drone was "conducting routine operations in the international waters of the South China Sea in full compliance with international law."
For its part, China said the handover of the small vessel was "completed smoothly" after "friendly consultations" between both sides, according to a short defense ministry statement on its website.
Pentagon officials had previously said the drone would be handed over to the crew of a US warship in the vicinity of Scarborough Shoal.
Pentagon officials said last week that the Chinese had "unlawfully" grabbed the marine probe, which they described as a craft that gathers unclassified data -- including water temperatures, salinity and sea clarity -- that can be used to help submarines navigate and determine sonar ranges in murky waters.
China said it snatched the craft because it might pose a safety hazard to other vessels. It also said it "strongly opposed" US reconnaissance activities and had asked Washington to stop.
The incident has heightened ongoing tensions in the South China Sea, where Beijing has moved to fortify its claims to the region by expanding tiny reefs and islets into artificial islands hosting military facilities.
Brunei, Malaysia, the Philippines, Vietnam and Taiwan have competing claims in the waterway.
While Washington takes no position on sovereignty claims in South China Sea, it has repeatedly called on China to uphold freedom of navigation.
Its military has conducted several operations in which ships and planes have passed near the sites Beijing claims.
US President-elect Donald Trump raised the rhetorical heat further last week, accusing Beijing of theft.
After Beijing and Washington announced the drone would be returned on Sunday, he tweeted: "We should tell China that we don't want the drone they stole back. - let them keep it!"
China's foreign ministry on Monday rejected Trump's accusations Beijing had stolen the craft as "not accurate."
The state-owned China Daily wrote in an editorial earlier that Trump's behavior "could easily drive China-US relations into what (US President Barack) Obama portrays as 'full-conflict mode'."
Trump, who is set to take office on January 20, had already infuriated Beijing by questioning longstanding US policy on Taiwan, calling Beijing a currency manipulator and threatening punitive tariffs against Chinese imports.