The present study was conducted to determine the crucial characters for sexing individuals of female and male pupae of the tomato leafminer Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae). The experiments were performed on 50 live pupae, which were examined under a stereozoom microscope and photographed. Two small tubercles present on the eighth abdominal segment of female pupae are helpful to distinguish the tomato leafminer from male pupae. The genital opening in females is found between the tubercles and is indicated by a longitudinal suture in the middle of the eighth abdominal segment. The male genital opening is also found on the ninth abdominal segment. Typically, female pupae and adults are heavier and bigger than males. This key information and pictures will be useful for laboratory studies that perform single pair matings, sex ratios, and other biological experiments.