© Northrop GrummanThe Manta Ray craft being towed in preparation for testing.
The prototype, which is several times larger than a small boat, completed full-scale testing off the coast of Southern California over the last three months.
US aviation giant Northrop Grumman made the drone with funding from the government's Defense Advanced Research Projects Agency (DARPA) programme.
The navy project aims to develop a vehicle that can roam the oceans for "very long periods of time" without supervision or the need to refuel.
The shape is inspired by the graceful glide of the Manta Ray, a social creature with a curious and playful nature, making it a favourite among scuba divers.
"Once deployed, the vehicle uses efficient, buoyancy-driven gliding to move through the water," says Dr Kyle Woerner, DARPA programme manager for Manta Ray.
Also known as unscrewed underwater vehicles (UUV), the drone can operate underwater without a human occupant and can anchor itself on the sea floor.
The Manta Ray's successful testing is seen as a milestone and aims to demonstrate a new class of long-duration UUV's ready for operations across seas and oceans, according to DARPA.
Comment: You can bet they didn't announce any unsuccessful effort: 2021: Satellite photo reveals wreckage of $2 BILLION US B-2 Stealth bomber, landing gear gave way after emergency landing - reports
"Our successful, full-scale Manta Ray testing validates the vehicle's readiness to advance toward real-world operations," Dr Woerner says.
Maritime drones are seen as a cost-effective way to boost defence capabilities while reducing risks to sailors.
The US is not the only country pressing ahead with UUV development. It joins both Russia and Ukraine in a technology race to dominate underwater warfare.
DARPA says it is engaging with the US Navy on the next steps for testing and the transition of UUV technology.
Whenever you see the name DARPA behind research, always keep in mind that DARPA does research for National Security. It is not a "peace" agency. It always Frames its work as protecting the military and the nation. Watch some videos of the former directors talking to the military; then you see it is about war and superiority of weapons. Who, you mght ask, is the enemy? Everyone.
What is covered under DARPA research? Water, crops, weather, AI, genetic modification, bioweapons, transhumanism, behavioral psychology, directed energy weapons, oh and the manta ray drone.