A K star is an orange-red star, of spectral type K. The spectra of K stars are dominated by the H and K lines of calcium and lines of neutral iron and titanium, with molecular bands due to cyanogen (CN) and titanium dioxide (TiO) becoming increasingly prominent at the cooler end of the range. K-type main sequence stars (that is, K-type dwarfs) are intermediate in size and temperature between M-type red dwarfs and Sun-like stars (type G), with a mass of 0.5 to 0.8 solar mass, a temperature of 3900 to 5200 K (3600 to 4900°C), and a luminosity 0.1 to 0.4 solar luminosity.

Nearby examples include Epsilon Indi, Epsilon Eridani, and Tau Ceti, the latter two having been the target stars of Project Ozma. Early type main sequence K stars within a few tens of light-years of the Sun are generally included in the list of target stars of searches for exoplanets and targeted SETI programs, since if they have planets orbiting within their habitable zones there is the possibility that these worlds support life of some kind. Giant K types are typically 100 to 400 K cooler, and have luminosities of 60 to 300 times that of the Sun and masses of 1.1 to 1.2 solar masses. Familiar examples of K-type giant stars include Arcturus (K1), Aldebaran (K5), and Pollux.