Spermatozoa are capable of producing small amounts of reactive oxygen species (ROS), and sperm in teratozoospermia generate more ROS than sperm in normozoospermia. The source of ROS production in ejaculated human sperm has not been fully clarified. Recently, NADPH oxidase 5 (NOX5) was detected in human sperm, and ROS generation by this enzyme was reported. We investigated the magnitude of NOX5 expression in normozoospermic (n = 12) and teratozoospermic (n = 13) semen samples with different percentages of abnormal sperm. The existence of NOX5 enzymes in sperm was analysed by immunocytochemistry and flow cytometry and correlated with morphological abnormalities. Immunofluorescent studies identified NOX5 in acrosomal, equatorial, post-acrosomal regions, the body and the tail of both normal and abnormal sperm. Teratozoospermic semen samples had higher percentages of NOX5-positive sperm and expressed more NOX5 (based on higher mean fluorescent intensity) than normal semen samples. Positive correlations were observed between abnormal sperm morphology and both the percentage of NOX5-positive sperm and the magnitude of NOX5 expression. Based on these findings, we can assume that there is a positive correlation between ROS generation in teratozoospermia and that in NOX5 expression.