I quantified relative growth of the walking legs of the Japanese mitten crab Eriocheir japonica (de Haan) in the Saigo River, Fukuoka Prefecture, Japan. By plotting the third walking leg merus length (LML) against carapace width (CW), I could distinguish four growth phases for both sexes; I then fitted a regression line of allometry (Log Y = a Log X b) to each phase. In the first phase (not sexed, CW < 4 mm), growth was positively allometric, and relative length of legs increased (a = 1.37). In the second phase (4 mm ≤ CW < 20 mm), growth was nearly isometric, and legs remained relatively longer (a = 1.05 for males and 1.04 for females). In the third phase (20 mm ≤ CW < 40 mm for male, and 20 mm ≤ CW < 37 mm for female), growth was negatively allometric, and relative leg length decreased (a = 0.82 for males and 0.70 for females). In the fourth phase (CW ≥ 40 mm for males and CW ≥ 37 mm for females) including adult crabs, growth was negatively allometric, and the legs become relatively shorter; for males, relative growth was more negative than for females (a = 0.82 for males and 0.92 for females). These patterns reflect the migratory habit in the life cycle of E. japonica. The first phase occurred just after settlement and metamorphosis (instars 1–3) before starting upstream migration and was preparatory for the migration. The second phase coincided with the period of upstream migration and dispersal. Relatively longer legs increase locomotor activity against the flow and help crab dispersion to the wide area along the river. The third phase is the growth phase after active migration until crabs attain maturity. The fourth phase is the maturity phase, during which adults participating in reproduction emerge and sexual dimorphism becomes evident. There was a morphological variation in fourth phase males (relatively long legs in smaller males and relatively short legs in larger males). The possibility of variation of mating strategy is discussed.