Sex chromosome aneuploidies are the most common chromosome abnormalities associated with infertility in adult men. 47, XYY syndrome also known as Jacob Syndrome (JS) is one sex chromosome aneuploidy. Majority of 47, XYY men show normal spermatogenesis while minority may have varying degrees of impairment in spermatogenesis. This case report discusses about a 32 year old Malay gentleman who was diagnosed to have azoospermia for which cytogenetic analysis revealed an abnormal mosaic 47,XYY/45,X karyotype pattern as the underlying genetic cause. Abnormal mosaic 47,XYY/45,X karyotype associated with infertility is extremely rare in human population and hence reported for its rarity.