A promising solution to solve the low phosphorus availability of most plant protein sources and make those ingredients more useful and functional in feeds for fish is reported. Three experimental diets supplemented with 0, 3, and 6% meju (designated as Meju 0, Meju 3, and Meju 6) were formulated. After 8 weeks of a feeding trial, growth performance, feed utilization, and survival were not different among fish groups fed the test diets. Phosphorus retention in fish fed Meju 6 diet was significantly higher than that of fish fed the control diet. Increased antioxidant activity in meju diets was observed with increasing polyphenol and flavonoids contents. Liver superoxide dismutase activity was significantly increased by the supplementation of meju. Microbial fermentation of soybean, which also can be simply adapted for soybean meal, is a promising method for increasing phosphorus availability in soybean meal, antioxidant activities in diets, and nonspecific immune response of olive flounder.