We analyzed the gonadal development and age of the spotted scat (Scatophagus argus) in the Matang Mangrove Forest Reserve, Malaysia. Histological analysis of gonads indicated that few females and males in the mangrove estuarine system were sexually mature, with few specimens having sizes greater than the typical maturation size (> 140 mm in total length for females, > 110 mm in total length for males). These observations suggested that S. argus do not mature sexually in the mangrove estuary. Growth analysis using age information from otolith increments suggested that S. argus moved or are carried into the mangrove estuary during the late tholichthys larval stage at a size of about 12 mm–14 mm in total length (ca. 37 days old), and that the fish use the estuary as a nursery ground before sexual maturation. The period of sojourn in the estuary is for a maximum of ca. 290 days for females by the size of ca. 160 mm–180 mm in total length and ca. 155 days for males by the size of ca. 110 mm in total length. After these nursery periods, grown specimens reaching sexual maturity are considered to leave the estuary habitats.