A polymerizable N-halamine precursor, 3-(4 '-vinylbenzyl)-5,5-dimethylhydantoin (VBDMH), was diffused into polyurethane (PU) in the presence of a cross-linker and an initiator. Post-polymerization of VBDMH led to the formation of PU/PVBDMH semi-interpenetrating polymer networks (IPNs). Upon chlorination, the amide groups in PVBDMH were transformed into stable N-halamines. The presence of N-halamines in the semi-IPNs was confirmed by ATR-IR and energy dispersive X-ray spectroscopy (EDS) analysis. The N-halamine contents in the semi-IPNs could be readily controlled by changing reaction conditions. EDS analysis showed that the N-halamines mainly stayed on the semi-IPN surface, rather than in the bulk, which could be caused by the incompatibility of VBDMH with the amorphous regions in the original PU. Contact mode antimicrobial tests and SEM observations demonstrated that the semi-IPNs had potent antimicrobial and biofilm-controlling effects against both Gram-positive and Gram-negative bacteria, pointing to great potentials of the new N-halamine-based semi-IPNs for a broad range of applications.