The vessel was located in international waters 70 miles off the coast of Delaware, heading north at 10 knots, according to one official. The US territory line is 12 nautical miles off the coast.
The ship, the SSV-175 Viktor Leonov, last sailed near the US in April 2015.
It is capable of intercepting communications or signals, known as SIGINT, as well as measuring US Navy sonar capability. It is also armed with surface-to-air missiles.
A US defence official said: “It’s not a huge concern but we are keeping our eyes on it.”
In the past, Russian spy ships have loitered off the coast of Kings Bay, Georgia, which is home to a US Navy ballistic missile submarine base.
During the Cold War, Russian intelligence gathering ships routinely parked off US submarine bases along the East Coast.