Kepler-22b (also known by its Kepler Object of Interest designation KOI-087.01) is an exoplanet orbiting within the habitable zone of the Sun-like star Kepler-22. It is located about 640 light-years (200 parsecs) from Earth in the constellation of Cygnus. It was discovered by NASA's Kepler Space Telescope in December 2011 and was the first known transiting planet to orbit within the habitable zone of a Sun-like star, where liquid water could exist on the planet's surface. The planet's host star Kepler-22 is too dim to be seen with the naked eye.

Kepler-22b's radius is roughly twice that of Earth. Its mass and surface composition are unknown. However, an Earth-like composition for the planet is believed to be unlikely; it is more likely to be an ocean planet or have a volatile-rich composition with a liquid or gaseous outer shell. The only parameters of the planet's orbit that are currently available are its orbital period (about 290 days) and its inclination (approximately 90°). Evidence suggests that the planet has a moderate surface temperature, assuming that the surface is not subject to extreme greenhouse heating. In the absence of an atmosphere, its equilibrium temperature (assuming an Earth-like albedo) would be approximately 279 K (6 °C; 43 °F), slightly higher than that of Earth's 255 K (−18 °C; −1 °F).

The planet's first transit was observed on 12 May 2009. Confirmation of the existence of Kepler-22b was announced on December 5, 2011.

Kepler-22b's radius was initially thought to be 2.4 times that of Earth, but has since been revised to 2.1 R_🜨 as of 2023^[update]. Its mass and surface composition remain unknown, with only some rough estimates established: at the time of the discovery announcement, it was known to have fewer than 124 Earth masses at the 3-sigma confidence limit, and fewer than 36 Earth masses at 1-sigma confidence. The adopted model in Kipping et al. (2013) does not reliably detect the mass (the upper limit is 52.8 M_🜨). As of 2023^[update], the upper limit has been constrained to at most 9.1 M_🜨.

Kepler-22b, dubbed by scientists as a "water world", might be an "ocean-like" planet. It might also be comparable to the water-rich planet Enaiposha although Kepler-22b, unlike Enaiposha, is in the habitable zone. An Earth-like composition is ruled out to at least 1-sigma uncertainty by radial velocity measurements of the system; it is thus likely to have a more volatile-rich composition with a liquid or gaseous outer shell; this would make it similar to Kepler-11f, one of the smallest known gas planets. Natalie Batalha, one of the scientists on the Kepler Space Telescope project, has speculated, "If it is mostly ocean with a small rocky core, it's not beyond the realm of possibility that life could exist in such an ocean". This possibility has spurred SETI to perform research on top candidates for extraterrestrial life.

In the absence of an atmosphere, its equilibrium temperature (assuming an Earth-like albedo) would be approximately 279 K (6 °C), compared with Earth's 255 K (−18 °C).

The host star, Kepler-22, is a G-type star that is 3% less massive than the Sun and 2% smaller in volume. It has a surface temperature of 5,518 K (5,245 °C; 9,473 °F) compared with the Sun, which has a surface temperature of 5,778 K (5,505 °C; 9,941 °F). The star is about 4 billion years old. In comparison, the Sun is 4.6 billion years old.

The apparent magnitude of Kepler-22 is 11.5, which means it is too dim to be seen with the naked eye.