Understanding radioactive Cs contamination has been a central issue at Fukushima Daiichi and other nuclear legacy sites; however, atomic -scale characterization of radioactive Cs in environmental samples has never been achieved. Here we report, for the first time, the direct imaging of radioactive Cs atoms using high -resolution high -angle annular dark -field scanning transmission electron microscopy (HAADF-STEM). In Cs -rich microparticles collected from Japan, we document inclusions that contain 27 - 36 wt% of Cs (reported as Cs 2 O) in a zeolite: pollucite. The compositions of three pollucite inclusions are (Cs 1.86 K 0.11 Rb 0.19 Ba 0.22 ) 2.4 (Fe 0.85 Zn 0.84 X 0.31 ) 2.0 Si 4.1 O 12 , (Cs 1.19 K 0.05 Rb 0.19 Ba 0.22 ) 1.7 (Fe 0.66 Zn 0.32 X 0.41 ) 1.4 Si 4.6 O 12 , and (Cs 1.27 K 0.21 Rb 0.29 Ba 0.15 ) 1.9 (Fe 0.60 Zn 0.32 X 0.69 ) 1.6 Si 4.4 O 12 (X includes other cations). HAADF-STEM imaging of pollucite, viewed along the [111] zone axis, revealed an array of Cs atoms, which is consistent with a simulated image using the multi -slice method. The occurrence of pollucite indicates that locally enriched Cs reacted with siliceous substances during the Fukushima meltdowns, presumably through volatilization and condensation. Beta radiation doses from the incorporated Cs are estimated to reach 10 6 - 10 7 Gy, which is more than three orders of magnitude less than typical amorphization dose of zeolite. The atomic -resolution imaging of radioactive Cs is an important advance for better understanding the fate of radioactive Cs inside and outside of nuclear reactors damaged by meltdown events.