XX male syndrome: A syndrome characterized by the presence of an XX sex chromosome complement in an individual with male genitalia including both testes but no sperm production (azoospermia).

Babies with the XX male syndrome may come to medical attention at birth because of hypospadias (urethral opening not at its normal position at the end of the penis) or because of cryptorchidism (undescended testes). However, most individuals with the syndrome have a normal penis and their testes in the scrotum and do not come to medical attention until after puberty and only then perhaps because of the small size of their testes, gynecomastia (breast development in an apparent male), or sterility resulting from the azoospermia.

Some but not all individuals with the syndrome have SRY, the gene normally found on the Y chromosome that makes for malesness. This is often due to an abnormal interchange between the Y chromosome and the X chromosome, resulting in presence of the SRY gene on the X chromosome and infertility. It may also be due to translocation of SRY to an autosome (a non-sex chromosome). The X/autosome translocation can occur de novo (be new) or be inherited from a parent.

The XX male syndrome is also called de la Chapelle syndrome after the Finnish physician Albert de la Chapelle who first described it.