Due to the advancements in the field of medical sciences, it has become possible to change one’s gender from male to female or vice versa. Sex change is a process by which a person or animal’s sex is medically reassigned or changed, i.e. female sexual characteristics and organs are substituted by male ones in order to change their sex, or vice versa. It is interesting to note that in some cases, sex change may occur naturally, for example, the case of the sequential hermaphroditism observed in some species. However, the sex of a human baby is determined by chromosomes and does not automatically change by itself, as far as it is observed so far. Males have a Y chromosome in addition to the X chromosome, while the females have two X chromosomes. The Y chromosome contains a specific gene which is a determining factor in terms of gender. This gene cause cells in an embryo to differentiate and develop male genitals. On the other hand, the embryos without the testes are the determining factor for the female gender. This is called gender dysphoria. However, the term is commonly used for sex reassignment therapy, including sex reassignment surgery, carried out on humans. It is also sometimes used for the medical procedures applied to intersex people. Islam does not permit for sex reassignment therapy in general. But in some cases where sex reassignment therapy is essential for which people facing gender identity disorder, this therapy is not only allowed but it is recommended also. In this article, we will discuss its basis and relevant rulings.