WASHINGTON—The Navy and its industry partners are “under cyber siege” by Chinese hackers and others who have stolen national security secrets in recent years, exploiting critical weaknesses that threaten the U.S.’s standing as the world’s top military power, an internal Navy review concluded.

The assessment, delivered to Navy Secretary Richard Spencer last week and reviewed by The Wall Street Journal, depicts a branch of the armed forces under relentless cyberattack by foreign adversaries and struggling in its response to...