The upper body undergarment is designed to be worn under a brassiere to provide support to a user's upper body by supporting a woman's breasts and underarms and constricting the loose parts of the upper body, without interfering with the specific desired appearance of the brassiere under outerwear. The upper body undergarment is made from hosiery material and includes tapered sides having sub-pectoral support portions that lift and support the breasts. The tapered sides are releasably attached to each other. The upper body undergarment may additionally include underarm support portions of the sleeves that are similar to the sub-pectoral support portions, and armpit panels made from breathable fabric attached between the shoulder portions and the sleeves at the armpit areas of the wearer to allow for greater ventilation. Strips of cloth are provided to attach the brassiere straps to the upper body undergarment. The bottom edges of the sleeves and the bottom edge of the torso area, in the front and the back, have extra elasticity for a more secure fit.