A Washington contractor, who has yet to be named, said its files on "secret plans to develop a supersonic anti-ship missile" had been hacked.
Hackers reportedly targeted a contractor which works for the Naval Undersea Warfare Center - a research and support centre in Newport, Rhode Island.
It said the stolen information amounted to 614 gigabytes of material, including secret plans to develop a supersonic anti-ship missile for use on US submarines by 2020.
The Post "agreed to withhold certain details about the compromised missile project at the request of the Navy, which argued that their release could harm national security". "It would be inappropriate to discuss further details at this time", the US Navy said in a statement to Reuters news agency.
He added: "There are measures in place that require companies to notify the government when a "cyber incident" has occurred that has actual or potential adverse effects on their networks that contain controlled unclassified information".
Unnamed US officials told the Post that hackers took 614 gigabytes of information from the contractor, which was also unnamed.More news: Just Cause 4 Leaks in Steam Ad
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"The breach is part of China's long-running effort to blunt the USA advantage in military technology and become the preeminent power in East Asia".
US officials believe China has for years carried out hacking attacks on the USA military, the USA government and US companies.
'They are completely focused on getting advanced weapons technology through all kinds of means. Sea Dragon is a $300 million project scheduled for underwater testing in September. The initiative, which is shrouded in secrecy, was first launched in 2012.
Other information stolen included signal and sensor data for submarines, information relating to cryptographic systems, and a Navy electronic warfare library. The report comes at a tricky time in US-China relations. The newspaper reported that while the data was housed on the contrator's unclassified network, when aggregated, officials said the data could be considered classified.
The investigation is being led by the Navy with the assistance of the FBI, officials said.