Black patches on the thighs may be caused by post-inflammatory hyperpigmentation, seborrheic keratosis, florid furuncle, melanoma and other diseases. 1. Post-inflammatory hyperpigmentation: post-inflammatory hyperpigmentation is usually caused by acute or chronic inflammation of the skin, and patients will have localized hyperpigmentation spots, mossy and other changes. If you have ever suffered from ringworm, it may lead to black spots on the thighs. 2. seborrheic keratosis: seborrheic keratosis is a benign tumor of the epidermis, the cause of which is related to skin aging, genetic susceptibility and other factors, occurring in the head, face, back of the hands, trunk, etc., but also in the thighs, manifesting itself in the size of the black spots. 3. florid chaff rash: a superficial fungal disease caused by Malassezia infection of the epidermal stratum corneum. The disease is chronic, with mild inflammation and usually no conscious symptoms. The damage is characterized by scattered or fused hypopigmented or hyperpigmented spots with furfuraceous flakes, which occur on the chest, back, upper arms, etc. and may also involve the thighs. 4. Melanoma is a tumor that results from the malignant transformation of melanocytes, the cause of which is related to DNA damage, and the triggers include sunburn, excessive exposure to ultraviolet rays, etc., and the patient’s skin will show the rapid enlargement of the original nevus, uneven color, and ulceration, etc., and it can occur on the legs. There are more reasons for black patches on the thighs, so it is recommended to consult a doctor for a clear diagnosis.