Many insects adjust feeding behavior to meet their optimal requirement for multiple nutrients. In the present study, we investigated the behavioral regulatory responses of male and female adults of an omnivorous beetle, Tenebrio molitor (Coleoptera: Tenebrionidae), using choice and no-choice feeding experimental designs. In the choice experiment, newly eclosed adults of both sexes received one of four food-pairing treatments and so were allowed to self-compose their preferred nutrient intake by selecting between two nutritionally-complementary diets (protein-rich vs. carbohydrate-rich diet; 35:7 vs. 7:35, 35:7 vs. 5.6:28, 28:5.6 vs. 7:35, or 28:5.6 vs. 5.6:28, P%:C% by dry mass). Our data showed that both sexes independently regulated their intake of protein and carbohydrate to a 1:1 ratio, indicating that they are capable of adjusting their feeding behavior to optimize their nutrient preference. In the no-choice experiment, adults were forced to eat one of seven single diets that varied in P:C ratio (0:42, 7:35, 14:28, 21:21, 28:14, 35:7, or 42:0). Results showed that both sexes ingested similar amounts of nutrients when the dietary P:C ratio was moderately balanced (14:28, 21:21 and 28:14), but males ingested significantly more than females on those diets that were extremely imbalanced with respect to their P:C ratio (0:42, 7:35, 35:7 and 42:0). This indicates that there exists a sex-specific difference in nutrient intake when the nutritional balance of diets deviates away from the optimal level under restricted feeding condition.