GJ 3929 is a cool M-dwarf (about 0.31 solar masses, 3,384 K, 0.32 solar radii), 15.8 light-years from Earth. GJ 3929 b is a 1.8-Earth-mass planet, with an orbital period of 2.6 days, at 0.025 AU from its star, and an equilibrium temperature near 568 K, discovered in 2022 via transit. GJ 3929 c is a 5.7-Earth-mass planet, with an orbital period of 15.0 days, at 0.081 AU from its star, and an equilibrium temperature near 317 K, discovered in 2022 via radial velocity. Mutual gravitational tugs between the planets can slowly reshape their orbits, preserving clues to the system's assembly.