The team, from the California Institute of Technology (Caltech), has no direct observations to confirm its presence just yet, BBC News reported.
Rather, the scientists make the claim based on the way other far-flung objects are seen to move.
But if proven, the putative planet would have 10 times the mass of Earth.
The group's calculations suggest the object orbits 20 times farther from the Sun on average than does the eighth - and currently outermost - planet, Neptune, which moves about 4.5 billion km from our star.
The Caltech astronomers have a vague idea where it ought to be on the sky, and their work is sure to fire a campaign to try to track it down.
"There are many telescopes on the Earth that actually have a chance of being able to find it," said Dr Mike Brown.
"And I'm really hoping that as we announce this, people start a worldwide search to go find this ninth planet."
The Caltech group has analysed the movements of objects in a band of far-off icy material known as the Kuiper Belt. It is in this band that Pluto resides.
The scientists say they see distinct alignments among some members of the Kuiper Belt - and in particular two of its larger members known as Sedna and 2012 VP113. These alignments, they argue, are best explained by the existence of a hitherto unidentified large planet.
"The most distant objects all swing out in one direction in a very strange way that shouldn't happen, and we realised the only way we could get them to swing in one direction is if there is a massive planet, also very distant in the Solar System, keeping them in place while they all go around the Sun," explained Dr Brown.
"I went from trying very hard to be sceptical that what we were talking about was true, to suddenly thinking, 'this might actually be true'."