Kepler-186f is almost exactly the same size as the Earth and occupies its parent star's "habitable zone", where temperatures are mild enough to allow liquid surface water.
If the planet has lakes or oceans, it would increase the chances of extraterrestrial life evolving there.
But anything living on the world may have to withstand extra large doses of radiation from its active sun.
The find is described in the journal Science as "a landmark on the road to discovering habitable planets".
Kepler-186 is an "M-dwarf" star 795 light years away that is smaller and cooler than the Sun.
It is orbited by a family of five known planets. Kepler-186f, the latest to be discovered, is the outermost.
Kepler-186f is just 0.1 times bigger than the Earth and while habitable zone planets have been identified around other stars, none of them so closely match the Earth in size.