Iron sulfide compounds are emerging as an important family of functional materials owing to their important properties and their applications in different technical fields. Well-defined Fe7S8 nanowires templated by thermal decomposition of [Fe16S20]/diethylenetriamine hybrid nanowires under an argon atmosphere are reported. As-prepared Fe7S8 nanowires show typical Michaelis–Menten kinetics and good affinity to both H2O2 and 3,3′,5,5′-tetramethylbenzidine. At pH 7.0, the constructed UV/Vis sensor showed a linear range for the detection of H2O2 from 0.5 to 150 μM with a correlation coefficient of 0.9998. The H2O2 sensor based on the Fe7S8 nanowires shows a highly sensitive response and has better stability than horseradish peroxidase when exposed to solutions with different pH values and temperatures. These excellent properties make the as-prepared Fe7S8 nanowires powerful tools for potential applications as an “artificial peroxidase” in biosensors and biotechnology.