Scientists say their initial studies suggest that the object - catalogued as V774104 - is some 500-1,000km across.
It will need to be tracked over time to learn the shape and extent of its orbit through the Solar System.
The discovery was announced at the 47th annual meeting of the American Astronomical Society`s Division for Planetary Sciences, taking place in National Harbor near Washington DC.
The team behind the find is led by Scott Sheppard, from the Carnegie Institution for Science, and Chad Trujillo, from the Gemini Observatory in Hawaii.
They specialise in detecting Solar System outliers.
In or out?
The previously recognised most distant object is the dwarf planet Eris. This body, which has a moon, Dysnomia, moves between 5.7 billion km and 14.6 billion km from the Sun.
To put some of these numbers in context: Earth is 149 million km from the Sun, and even the most distant major planet - Neptune - seems close at 4.5 billion km, by these standards.
That said, the Voyager 1 probe is further away - just. The epic robotic explorer has now ventured 20 billion km from home.
The big question is whether V774104 sweeps inwards from its present location, like Eris, or outwards, like the objects known as 2012 VP113 and Sedna.
Unexplained history
These bodies are currently slightly closer in than Eris, but investigations of their orbits show they will reach far deeper into space, out to 66 billion km and 140 billion km, respectively.
Models for Solar System formation suggest that such objects were probably not created in these weird, eccentric orbits.
One explanation is that they have been perturbed gravitationally and pulled on to their strange trajectories by a passing planet - perhaps one that was expelled from our Solar System early in its history.
Some scientists even speculate that such objects could have been stolen from a star that formed from the same "nursery" of gas and dust as our Sun 4.6 billion years ago.
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