Kepler-138, also known as KOI-314, is a red dwarf located in the constellation Lyra, 219 light years from Earth. It is located within the field of vision of the Kepler spacecraft, the satellite that NASA's Kepler Mission used to detect planets transiting their stars.

The star hosts three confirmed planets and a likely fourth, including the lowest-mass exoplanet with a measured mass and size discovered to date, Kepler-138b, with a mass comparable to that of Mars. Kepler-138d is remarkable for its low density; initially thought likely to be a gas dwarf, more recent observations as of 2022 show that it, as well as planet c, are likely to be ocean worlds.

Prior to Kepler observation, KOI-314 had the 2MASS catalogue number 2MASS J19213157+4317347. In the Kepler Input Catalog it has the designation of KIC 7603200, and when it was found to have transiting planet candidates it was given the Kepler object of interest number of KOI-314.

Planetary candidates were detected around the star by NASA's Kepler Mission, a mission tasked with discovering planets in transit around their stars. The transit method that Kepler uses involves detecting dips in brightness in stars. These dips in brightness can be interpreted as planets whose orbits pass in front of their stars from the perspective of Earth, although other phenomena can also be responsible which is why the term planetary candidate is used. By timing these dips, gravitational interactions were detected between two of the candidates, allowing for a measurement of their masses and confirmation as real planets given that the masses were significantly below the deuterium burning limits.

Following the acceptance of the discovery paper, the Kepler team provided an additional moniker for the system of "Kepler-138". However, the planets were discovered by scientists outside of the Kepler team who referred to the star as KOI-314, as the Kepler designation had not been assigned yet.

Candidate planets that are associated with stars studied by the Kepler Mission are assigned the designations ".01", ".02", ".03", etc. after the star's name, in the order of discovery. If planet candidates are detected simultaneously, then the ordering follows the order of orbital periods from shortest to longest. Following these rules, the first two candidate planets were detected simultaneously and assigned the names KOI-314.01 and KOI-314.02, with respective orbital periods of 13.8 and 23.1 days. Over a year later, a much smaller planet candidate was detected and assigned the name KOI-314.03, despite being the shortest orbital period planet (period of 10.3 days) found in the system.

Confirmed planets are conventionally assigned the designations b, c, d, etc. after the star's name. The labels are assigned alphabetically in the order of discovery starting from b. Since KOI-314.01 and KOI-314.02 were confirmed as planets simultaneously, the alphabetical names were assigned in order of orbital period by the discoverers, and thus became KOI-314b and KOI-314c respectively. Since no gravitational interactions were detected due to KOI-314.03, this planetary candidate remained unconfirmed as 6 January 2014 and thus kept the same name.

In the following weeks, on 28 February 2014, a new paper validated KOI-314.03 as being a real planet with a false alarm probability of less than 1%. The new paper used different names for the planets, going from KOI-314b to Kepler-138c, KOI-314c to Kepler-138d and KOI-314.03 to Kepler-138b. These designations have been used by subsequent studies, and by databases such as the NASA Exoplanet Archive. This situation is similar to that of some other planetary systems such as Mu Arae, where different designations have been used for the same planets in the literature.