Police in the Australian state of Victoria reported their mysterious visitor in a Facebook post, revealing that those that found the craft were "stunned as mullets" by the discovery.
The Saildrone was launched from Bluff in January 2019 on a mission to research the krill population around Antarctica.
A US firm set three unmanned Saildrones off on the mission, using sophisticated sounding equipment to track the tiny crustaceans.
But one went rogue and the firm lost contact with it somewhere between Antarctica and Cape Horn, at the tip of South America.
With a fixed hard sail, motor, rudder and keel, the boat is designed to run autonomously and Victoria Police said only one had ever gone missing.
It was found by surf lifesavers at Waratah Bay in Victoria, covered in barnacles.
Waratah Bay Marine Rescue senior skipper Greg Mouldings told the ABC he found the drone at 6am Tuesday.
"One of the rescue-crew mentioned his sister-in-law found something funny on the beach," he said.
"He showed me a photo and I was like, 'oh my god, what is that?' I've never seen anything like this in my life.
"I went down to the beach and was like, 'what is this?' I was expecting a wheelbarrow size, but it's 8 metres long. It's not budging anytime soon."
Mouldings said he had spoken with a researcher in the US who was instructing him on what to do next.
"The bloke in San Francisco is chuffed. He's given me instructions on how to dismantle it," he said.
"The scientist reckons it hit an iceberg two years ago around the bottom of South America and they lost connection.
"It's done thousands and thousands of clicks just sailing around the world on its own. And here it is still intact.
"It's a clever little boat."
