Density functional theory calculations are performed to investigate the structural, electronic and optical properties of Cu2ZnSnSe4 alloys. The lattice constants linearly decrease with the increment of the Si concentration and the mean atomic weight. The Cu2ZnSixSn1-xSe4 alloys are miscible with low formation enthalpies. A tunable bandgap is obtained in a wide energy range (1.06-2.26 eV) and primarily due to the upward shift of the conduction band minimum. The alloy Cu2ZnSi0.5Sn0.5Se4 has an optimal bandgap (1.49 eV) and a higher optical absorption coefficient than that of the well-studied CuZnSnS4. The bandgap-tunable Cu2ZnSixSn1-xSe4 alloys can be promising for their photovoltaic applications. (C) 2018 Published by Elsevier B.V.