Crosslinked polythiosemicarbazide (PTSC) membranes with a positively charged surface are fabricated via a reaction with (3-glycidyloxypropyl)trimethoxysilane. The integrally asymmetric ultrafiltration membranes discussed here can be easily prepared by water-induced phase separation using a PTSC solution in dimethylsulfoxide (DMSO). The crosslinked PTSC membranes are stable in DMSO, N,N-dimethylformamide, and tetrahydrofuran and they reject molecules of molecular weights (MW) above 1300 g mol−1. The influence of the crosslinking agent on the surface charge, membrane solvent resistance, and membrane performance is discussed. The crosslinked asymmetric PTSC membranes totally reject Direct Red dye (MW 1373 g mol−1), while the pristine PTSC membrane does not show any rejection for this dye. This finding suggests that an inorganic-type-network is formed during the crosslinking reaction, which tunes the pore size of the prepared membranes.