Over the past three decades, astronomers have discovered that small, rocky planets in temperate orbits are common around our neighboring stars. Whether or not these planets can maintain atmospheres is an open question in the field. Recently, considerable attention in this search has been focused on M dwarfs stars, as their small sizes make it easier to detect and characterize planets. However, M dwarfs, especially those with the lowest masses, exhibit more intense and longer lasting magnetic phenomena than Sun-like stars, emitting frequent flares which may erode planetary atmospheres over time. Infrared observations have ruled out thick atmospheres on several rocky worlds around low-mass M dwarf stars, casting doubt on whether or not rocky planets around M dwarf stars can maintain their atmospheres. We report radial velocity observations of TOI-700, an M2.5 dwarf star that hosts four small planets. We find that TOI-700 c (2.6 REarth) is a warm, low-mass (2.5 MEarth) planet with a H/He atmosphere. Furthermore, the small (1.07 REarth), temperate planet, TOI-700 d, is likely iron rich and/or rocky. TOI-700 c's H/He dominated atmosphere demonstrates that the star's irradiation did not completely eradicate the planet's primordial atmosphere. Because the Habitable Zone planets, TOI-700 e and d, orbit further from the star, they experience less activity than TOI-700 c – a level evidently insufficient to erode a lightweight atmosphere. This suggests that heavier, more Earth-like atmospheres expected around TOI-700 e and d might survive as well. TOI-700 demonstrates that rocky planets around high mass M dwarfs can maintain atmospheres despite flares in the early portions of these stars' lives.