Tinea pedis (TP) may lead to the development of foot ulcers in diabetic patients; thus, its prevention in diabetic patients is important. TP occurs after dermatophytes on the skin scales of TP patients attach to the feet. Therefore, it is necessary to remove the scales and dermatophytes, and this can be performed using various methods, including foot washing. This study aimed to objectively examine the association between the presence of TP and foot-washing habits. We included 33 diabetic patients, and, of these, 17 had TP. The presence of washing residue on the feet was determined by applying a fluorescent cream to the participants' feet, and images of the feet were captured under ultraviolet light before and after foot washing. Our results showed that diabetic patients with TP had higher levels of washing residue on their feet than those without TP. The importance of washing feet to prevent TP needs to be emphasized through educational programs for diabetic patients. Furthermore, the development of an effective foot-washing technique is essential.