Kepler-399 is a Sun-like G-type star (about 0.90 solar masses, 5,502 K, 0.68 solar radii), 741 light-years from Earth. Kepler-399 b is a planet with 1.0 Earth radii, with an orbital period of 14.4 days, at 0.103 AU from its star, discovered in 2014 via transit. Kepler-399 c is a planet with 1.4 Earth radii, with an orbital period of 26.7 days, at 0.155 AU from its star, discovered in 2014 via transit. Kepler-399 d is a planet with 1.9 Earth radii, with an orbital period of 58.0 days, at 0.261 AU from its star, discovered in 2014 via transit.