In this study, the reproductive biology of the silver stripe blaasop Lagocephalus sceleratus (Lessepsian pufferfish)from the Mediterranean waters, Egypt was investigated during 2014 using 795 specimens in order to fill the current gap in knowledge of such important species and the relation to its rapid distribution and impact on its new ecosystem. The results revealed that the percentage of males was slightly higher than that of females, representing 52.44% of all individuals, with a sex ratio of 1:0.91 (M: F). Fish length at first sexual maturity (L_m=50% maturation) was lower in males (38.5cm) than females (41cm). The elevation of gonadosomatic index (GSI) indicates that the spawning season extended from April to September. The ripe ova diameter was ranged from 0.5 to 0.82 mm, which occurred during the spawning period with a clear one peak towards the last months of spawning. Absolute fecundity increased as the fish grew in length and weight; it ranged from 260288 to 2372931 with an average of 1449522±720975. The relative fecundity ranged from 5784 to 32,957 for fish lengths ranging from a group (43-45) to (70-72 cm), with an average of 24,478 ±10,011, while it ranged from 698 to 3285 g, with an average of 776±231eggs for fish weight range (373 to 722 g). Histologically, this species is considered a totally or unispawner, spawns a synchronous group, and releases the ripe ova in one batch during the spawning season. The present results reflect the successful reproduction and high fecundity, explaining its success and rapid spread of L. sceleratus in its new ecosystem of the Mediterranean Sea companied with the social impact on the fisheries community and probabilities of toxic cases occurrence. This work is recommended to be used in further studies on pufferfish species, which need intensive attention in different scientific fields for various purposes.