The "highly sensitive data" was held by the contractor in a network of unclassified information, triggering concerns about possibly lax policing of contractor security protections, according to the Post.
Other information stolen included signal and sensor data for submarines, information relating to cryptographic systems, and a Navy electronic warfare library.
Plans included an anti-ship missile system to be installed on United States submarines by 2020.
The investigation into the breach, which is being jointly conducted by the Navy and the FBI, is now underway, The Post reported.
The breaches took place in January and February, US officials told the Washington Post, speaking on condition of anonymity about an ongoing investigation led by the Navy and assisted by the Federal Bureau of Investigation.
Navy spokesman Commander Bill Speaks declined to confirm the Post report, citing security reasons.More news: Thousands gather for 'Trooping the Colour' for Queen Elizabeth
She said it would be "inappropriate to discuss further details at this time".
Former Sen. Jim Talent (R-Mo.), who is a member of the U.S. China Economic and Security Review Commission, called the incident 'very disturbing, ' adding however that reports of the Chinese hacking scheme was nothing new.
At the Navy's request, the Post withheld information about the compromised new missile system, but said it was for a supersonic anti-ship missile that could be launched from submarines. The news comes as President Donald Trump's administration is seeking to secure Beijing's support in persuading North Korea to give up nuclear weapons, even as tensions persist between the U.S. and China over trade and defence matters.
The cyber attacks were part of China's effort to become the dominant sea power in the Asian Pacific, and come as Presideent Trump has touted his relationship with Chinese President Xi Jinping, whom the president is relying on to help negotiate an end to North Korea's nuclear weapons program.
The little information the Pentagon has released about Sea Dragon says the program has the capacity to introduce "disruptive offensive capability" by 'integrating an existing weapon system with an existing Navy platform'. Sea Dragon is a $300 million project scheduled for underwater testing in September.