Exercise-associated hyponatremia (EAH) is a life-threatening dilutional hyponatremia that typically occurs during or immediately after exercise in endurance athletes. A 49-year-old actress experienced dizziness 15 min after a 2-h stage performance while drinking several bottles of water. Thirty minutes later, the patient fell unconscious and was hospitalized. On admission, she showed dilutional hyponatremia (117 mmol/L) with extremely elevated arginine vasopressin (11.3 pg/mL). After initial treatment with 3% saline, her sodium levels immediately increased, and she recovered consciousness without developing subsequent osmotic demyelination syndrome. This case emphasizes the need for caution against excessive fluid intake during and/or after exercise to avoid EAH, even in non-athletes.