LTT 3780 is a cool M-dwarf (about 0.38 solar masses, 3,358 K, 0.38 solar radii), 22.0 light-years from Earth. LTT 3780 b is a 2.5-Earth-mass planet, with an orbital period of 18.4 hours, at 0.012 AU from its star, and an equilibrium temperature near 903 K, discovered in 2020 via transit. LTT 3780 c is a 8.0-Earth-mass planet, with an orbital period of 12.3 days, at 0.076 AU from its star, and an equilibrium temperature near 359 K, discovered in 2020 via transit. Mutual gravitational tugs between the planets can slowly reshape their orbits, preserving clues to the system's assembly.