ABSTRACT In tetrapods, body weight (BW) is a reliable predictor of extinction risk as it is representative of their life cycle, physiology, and ecology: low BW species tend to be less vulnerable compared to larger ones. In marine fish, excepting elasmobranchs, sturgeon, and salmonids, this relationship is not statistically significant; and in marine invertebrates it is unknown. In this study, the BW was evaluated as a predictor of extinction vulnerability in marine invertebrates at two taxonomic levels, assuming that endangered species lists indicate true extinction risk. At the order level, a correlation was performed between BW and the proportion of species in conservation lists concerning the total number of species (TS) of 17 orders. At the species level, we compared the average BW of listed versus not listed species by fitting a logistic regression between the BW and the presence/absence of species in these lists. We found no relationship between TS and BW, but there was a significant difference in the BW of listed versus not listed species. The relationship between the BW and the presence/absence of species in conservation lists was weaker in marine invertebrates compared to that in tetrapods and fish. The BW is an unreliable predictor of extinction risk in marine invertebrates. Thus conservation efforts should focus on maintaining and restoring the microhabitat of invertebrate species.