Understanding how growth rates are influenced by environmental conditions is important for proper management of commercially harvested species, especially under current and predicted future conditions of global climate change. Blue crabs are of high commercial value in the United States, but are declining in number throughout their range. Previous laboratory and field studies suggest that temperature and salinity play a role in blue crab growth rates, although the results of these studies have been inconclusive and, in some cases, contradictory. Furthermore, these studies have focused primarily on later juvenile stages [>20 mm carapace width (CW)]. This study examined the effects of temperature and salinity on early juvenile (∼2.5–16 mm CW) blue crab Callinectes sapidus growth rates. Crabs were reared in the laboratory under six different temperature and salinity treatments from the megalopal stage until the sixth juvenile instar. Compared with crabs reared at lower temperature, crabs reared at higher temperature exhibited a decreased intermolt period (IMP) and also decreased growth per molt. Salinity had no significant effects on IMP or growth per molt at 20°C and only slight effects on IMP at 30°C. These results suggest that temperature plays a major role in determining blue crab growth rates during the early juvenile instars, but the effects of salinity are relatively minor. Given forecasted increases in temperature throughout the geographic range of the species, these results suggest that blue crabs will reach maturity faster in the future, but will be smaller in size. Future studies should investigate potential causes of the correlation between salinity and blue crab size, and consider additional environmental influences.