Structural and electronic properties of the - and -phases of cerium sesquisulfide, Ce2S3, are examined by first-principles calculations using the GGA+U extension of density functional theory. The strongly correlated f-electrons of Ce are described by a Hubbard-type on-site Coulomb repulsion parameter. A single parameter of eV yields excellent results for crystal structures, band gaps, and thermodynamic stability for both Ce2S3 allotropes. This approach gives insights in the difference in color of brownish-black -Ce2S3 and dark red -Ce2S3. The calculations predict that both Ce2S3 modifications are insulators with optical gaps of 0.8 eV (-phase) and 1.8 eV (-phase). The optical gaps are determined by direct electronic excitations at from localized and occupied Ce 4f-orbitals into empty Ce 5d-states. The f-states are situated between the valence and conduction bands. The difference of 1 eV between the optical gaps of the two Ce2S3 modifications is explained by different coordinations of the cerium cations by sulfur anions. For both Ce2S3 modifications the calculations yield an effective local magnetic moment of 2.6  per cerium cation, which is in agreement with measurements. The electronic energy of the -phase is computed to be 6 kJ mol−1 lower than that of the -phase, which is consistent with the thermodynamic stability of the two allotropes.