A new superionic Ag+ conductor with a nominal composition Ag17(CO3)3I11 (11AgI-3Ag2CO3) was found in the AgI-Ag2CO3 system. The conductor, which was formed at temperatures from 100 to 170 °C, is a metastable phase that gradually decomposes into AgI and Ag10(CO3)I4 over a period of a few weeks at room temperature (RT). A Ag+ ionic conductivity of 0.16 S/cm was measured at RT, and an activation energy of 0.33 eV was evaluated in the temperature range from -9 to 19 °C. Single-crystal X-ray analysis revealed that Ag17(CO3)3I11 crystallized in a rhombohedral unit cell with hexagonal parameters of a = 15.8831(6) Å and c = 30.0730(13) Å at -183 °C and space group R3c. The Ag atoms were distributed over 53 sites in the asymmetry unit, with a maximum occupancy of 0.33(8). The continuous distribution of the partially occupied Ag sites was associated with the conduction paths of the Ag+ ions in the structure.