GJ 3998 is an orange K-type dwarf (about 0.52 solar masses, 3,726 K, 0.50 solar radii), 18.1 light-years from Earth. GJ 3998 b is a 2.5-Earth-mass planet, with an orbital period of 2.7 days, at 0.030 AU from its star, discovered in 2016 via radial velocity. GJ 3998 c is a 6.8-Earth-mass planet, with an orbital period of 13.7 days, at 0.090 AU from its star, discovered in 2016 via radial velocity. GJ 3998 d is a 6.1-Earth-mass planet, with an orbital period of 41.8 days, at 0.189 AU from its star, discovered in 2025 via radial velocity.