Trichophyton rubrum (T. rubrum) is the most common causative agent of dermatophytosis worldwide. The development of antifungal drugs will contribute to treating the disease. In this study, we suggest that a hemiprotonic compound phenanthroline-phenanthroline⁺ (ph-ph⁺) is active in inhibiting the growth and reproduction of T. rubrum, and the minimum inhibitory concentration and minimum fungicidal concentration values were 2 μg/ml and 8 μg/ml, respectively. In an in vitro onychomycosis model, ph-ph⁺ killed T. rubrum by inducing apoptosis, which was evaluated by transmission electron microscopy and Annexin V-FITC/propidium iodide staining. Transcriptomic analysis and biochemical assay showed that ph-ph⁺ elevated iron ion content in T. rubrum cells and reduced glutathione antioxidant system level, leading to an increase in the contents of ROS and malondialdehyde. Therefore, the antifungal mechanism of ph-ph⁺ would be associated with iron ion–induced cell apoptosis, which is different from other known antifungal drugs. Furthermore, ph-ph⁺ was prepared into gel for application in guinea pigs with dermatophytosis caused by T. rubrum. The results showed that the ph-ph⁺ gel eliminated the fungus in the animals without causing skin irritation or other adverse reactions. The study would not only provide a potential compound to treat dermatophytosis, but also suggest that iron ion–induced cell apoptosis might be a new approach to killing fungi.