In the Mexican Caribbean and southern Gulf of Mexico (GOM), the lionfish (Pterois volitans) was detected late in 2009 where now this fish is considered established. Its reproductive potential is among the most important factors driving its invasive success in the region; thus, understanding its ecology and biology, and in particular its reproductive aspects, are key in attempting counterbalancing its population increase in the region by culling. This work presents the reproductive aspects of lionfish in the Parque Nacional Arrecife Alacranes, a Mexican protected area in the southern GOM, off the northern Yucatan Peninsula. A total of 368 lionfish were obtained from lobster fishermen in 2012, and from 2014-2018. Based on gonad histological analyses, there were 18% females (N = 65) and 82% males (N = 303) (F:M = 0.21:1) where females showed a size range of 139 to 390 mm TL and males 183 to 440 mm TL. While sex proportion was skewed to males, the size class of 236 mm TL was not significantly different to a sex proportion close to equilibrium (F:M = 0.82:1). Active females were found in May and December, and there were spawning peaks from July to December. Maturation size for females and males were 160 mm and 207 mm TL, respectively. Size at which 50% females and males reach sexual maturity is 181 and 201 mm TL, respectively. These results are new contributions in describing the reproductive biology of lionfish in the southern GOM. These are compared with results found in the northern GOM.