A Chthonian planet (sometimes misspelled Cthonian), is a gas giant with its hydrogen and helium atmosphere stripped away due to its closeness to its star. The remaining rocky or metallic core would resemble a terrestrial planet in many respects.

Osiris is an example of a planet that is in the process of having its atmosphere stripped away, though it is not itself a Chthonian planet nor is it expected to become one in the near future. Some scientists suggest that Mercury is a Chthonian planet, but other than this possibility no known Chthonian planets have been discovered.

The word chthon (as in chthonic deity ) is from Chthonia , which means "Of the Earth" in Greek. There are at least three different Chthonias in Greek mythology, and they are all closely associated with the goddess Demeter.