ABSTRACT: Absent natural population control, invasive red lionfish Pterois volitans (hereafter, lionfish) have reached record densities in the northern Gulf of Mexico (nGOM), though the role of density dependence on their population dynamics remains poorly understood. This study examined the effects of population density, sex, and habitat on lionfish condition (i.e. mass relative to total length) and size-at-age. Lionfish density was estimated with a remotely operated vehicle during 2010-2017 at a series of nGOM natural (n = 16) and artificial (n = 22) reefs, and individual lionfish (n = 3296) were sampled at additional reefs in the same system between 2013 and 2017. Mean lionfish total length increased across time, while density increased through 2015 and then stabilized or slightly declined. Lionfish density at artificial reefs was 2 orders of magnitude greater than at natural reefs throughout the study. Fish condition was lower on artificial reefs across all years, and lower on natural reefs during 2015-2017 versus 2013-2014. Age estimates obtained from sagittal otoliths ranged from 0.2-7.7 yr, corresponding to birth years between 2008 and 2016. There were significant differences in growth and size-at-age between sexes and habitats, with males attaining larger sizes-at-age than females and fish growing faster at natural reefs. Significant declines in mean size-at-age and condition as a function of lionfish density were also observed. Overall, these results indicate condition and size-at-age displayed density-dependent effects that were likely due to inter- and intra-specific competition, which has important implications for invasive lionfish carrying capacity in the nGOM.