Identifying suitable alternatives to fish oil for inclusion in formulated diets for aquaculture species is becoming increasingly important; however, relatively few studies have assessed the potential of terrestrial oils as possible replacements for fish oil in diets for marine crustaceans. This study examined the potential of soybean oil as a partial or complete replacement for fish oil in diets for blue swimmer crab Portunus pelagicus juveniles. Seven iso-nitrogenous and iso-lipidic diets were formulated to contain soybean oil to fish oil ratios of 1:0. 3:1. 2:1. 1:1. 1:2. 1:3. and 0:1. They were fed to first-stage crabs over four molts and survival, development, and growth of the crabs were measured. Crabs fed the diet containing soybean oil as the sole lipid source (i.e., soybean oil:fish oil ratio 1:0 treatment) recorded the lowest survival among treatments, significantly longer intermolt duration, and significantly lower specific growth rates compared with other treatments. Improved performance was shown by crabs fed diets containing fish oil at the same, or higher, dietary level than soybean oil but the best survival, shortest intermolt duration, and fastest growth rates were recorded for crabs fed the diet containing the soybean oil:fish oil ratio of 1:1. The results indicated that P. pelagicus juveniles require a balance of dietary n-3 and n-6 fatty acids and thus dietary lipid requirements can be met partially by soybean oil. Diets containing soybean oil are cheaper to produce than traditional fish oil—based diets and soybean oil is a more sustainably sourced ingredient than fish oil. Our results should support further development of the P. pelagicus aquaculture industry.