© 2015 American Chemical Society. We report the synthesis and photocatalytic and magnetic characterization of colloidal nanoheterostructures formed by combining a Pt-based magnetic metal alloy (PtCo, PtNi) with Cu2ZnSnS4 (CZTS). While CZTS is one of the main candidate materials for solar energy conversion, the introduction of a Pt-based alloy on its surface strongly influences its chemical and electronic properties, ultimately determining its functionality. In this regard, up to a 15-fold increase of the photocatalytic hydrogen evolution activity was obtained with CZTS-PtCo when compared with CZTS. Furthermore, two times higher hydrogen evolution rates were obtained for CZTS-PtCo when compared with CZTS-Pt, in spite of the lower precious metal loading of the former. Besides, the magnetic properties of the PtCo nanoparticles attached to the CZTS nanocrystals were retained in the heterostructures, which could facilitate catalyst purification and recovery for its posterior recycling and/or reutilization.