Main Causes of Dark Inner Thighs Friction and Chafing: Constant rubbing of the inner thighs against each other (often linked to obesity or exercise) or against tight clothing. Hormonal Imbalance: Fluctuations during pregnancy, menstruation, or due to conditions like Polycystic Ovary Syndrome (PCOS). Acanthosis Nigricans: A skin condition that causes dark, velvety patches, frequently associated with insulin resistance and diabetes. Post-inflammatory Hyperpigmentation (PIH): Darkening that occurs