The main objective of this systematic review was to examine the effectiveness of protein supplementation through diet or dietary supplements on osteoporosis in postmenopausal women as evidenced by randomized controlled trials (RCTs). Five RCTs were included using dietary protein (N = 2), protein supplements (N = 2), and proteins through diet and supplements (N = 1). A total of 677 postmenopausal woman were included, all diagnosed with osteoporosis (T score < -2.5) and aged between 50 and 80 years. Results have found that combined protein administration through diet, mainly from animal sources and supplemental proteins (whey proteins, 86 g/d PRO including 6 g WPI), for a short period of time (up to 12 months) may positively affect osteoporosis in postmenopausal women. In addition, a positive effect can also be achieved by the single administration of a 250 mg/d supplement in which 10 g was WPI for a six-month period. In this review, it is shown that both combined administration of proteins through diet and supplements and single administration through protein supplements may reduce the risk of fracture in postmenopausal osteoporotic women. In contrast, dietary proteins alone, in doses similar to and/or higher than the RDA values, may not have any positive effect on treating osteoporosis.