This article presents a closed circular multipass cell (CMC) platform to measure terahertz (THz) atmospheric attenuation under various environmental conditions. The system provides a 16.61 m propagation path with 26 internal reflections in a compact chamber. This design overcomes spatial constraints of long-path experiments and sensitivity limitations of small cells. Measurements in the 330–490 GHz band demonstrated high agreement with the ITU-R P.676-13 model under precisely controlled relative humidity (20–90%, ±0.5%) and facilitates reliable measurements with high attenuation levels which are difficult to achieve in open air environments. The CMC platform is especially useful as a key tool for future 6G channel modeling and atmospheric propagation research.