Osteoporosis is an asymptomatic, systemic bone disease characterized by low bone mass and microarchitectural deterioration of bone tissue, resulting in increased bone fragility. Such condition is often underdiagnosed and undertreated, especially in men, therefore considerably increasing the fracture risk. Of note, fracture-related morbidity and mortality is generally higher in men, partly due to greater frailty. On the other hand, male hypogonadism is defined as the failure of the testes to produce androgens, sperm, or both and it is often due to the ageing process. This disorder, in turn, causes many systemic disorders, and it is the condition mainly associated with male osteoporosis. Testosterone replacement therapy (TRT) is usually prescribed to restore optimal hormone levels, but conflicting data are available about the efficacy of TRT treatment on bone mineral density. In this review we extensively examined literature data about the usefulness of TRT in improving hypogonadism-associated low bone mineral density. Furthermore, we considered the complex relationship between male osteoporosis and hypogonadism, by specifically addressing the role of androgens in male bone physiology and the diagnostic approach to male osteoporosis and hypogonadism and also by dealing with some new related aspects such as the new endocrine pathways between bone and testis and the role of androgen receptor CAG polymorphism on bone density.