Kepler-138 is an orange K-type dwarf (about 0.54 solar masses, 3,841 K, 0.54 solar radii), 66.9 light-years from Earth. Kepler-138's outer planets are candidate water worlds: low densities that imply deep oceans beneath thick atmospheres. Kepler-138 b is a 0.1-Earth-mass planet, with an orbital period of 10.3 days, at 0.075 AU from its star, and an equilibrium temperature near 452 K, discovered in 2014 via transit. Kepler-138 c is a 2.3-Earth-mass planet, with an orbital period of 13.8 days, at 0.091 AU from its star, and an equilibrium temperature near 410 K, discovered in 2014 via transit. With 4 confirmed worlds, Kepler-138 is a densely packed system whose resonances, spacing, and formation history are active research topics.