The invasive sea slug Pleurobranchaea maculata has quickly spread along the southwestern Atlantic coast, expanding almost 1500 km from Mar del Plata (Buenos Aires) to Bah & iacute;a Camarones (Chubut) in approximately six years. Certain features of its reproductive biology are of fundamental importance to understand its establishment and expansion. The present work provides a detailed description of the egg masses and intracapsular embryonic development of P. maculata in its non-native environment, described by means of field observations and microscopy (optical and scanning electron) techniques. Aquarium experiments to induce oviposition and to evaluate the effect of temperature on embryonic development were conducted to provide information on the potential invasion range of this species. The egg mass structure and peripheral distribution of embryos within it enable larger egg masses and, consequently, maximizing recruitment. Egg masses were observed in the field from July to January, increasing in number with rising daylight hours (maximum in summer). An experimental estimation showed that each individual can spawn at least a mean of five egg masses in an annual light cycle, representing, at least, more than half a million embryos per individual during a year. Significantly larger embryonic shell length and shorter embryonic developmental times were found at higher seawater temperatures. The limits of embryonic thermal tolerance during intracapsular development suggest P. maculata may continue extending its distribution range to the south. The comprehension of the reproductive strategies of the species in its invaded region favours the understanding of the invasive capacity of this potentially toxic sea slug.