The present study is an attempt for assessing the relationship between iron deficiency and discoloration in Undaria pinnatifida sporophytes. First, thallus discoloration and the decrease in photosynthetic pigment contents were examined in Undaria pinnatifida by employing an indoor iron-deficiency experiment. When Undaria pinnatifida sporophytes were cultured under iron-deficient conditions, which the iron concentrations were between 2 and 5 μg/L, the growth decreased, the thalli became discolored, and the photosynthetic pigment contents decreased. Iron deficiency caused the sporophytes to become lighter (L*) and less red (a*). Therefore, it appears that iron deficiency induces the discoloration of Undaria pinnatifida sporophytes, similar to how nitrogen and phosphorus deficiencies perform. Next, we investigated how the mechanism involved in the addition of iron affects the discolored Undaria pinnatifida sporophytes due to iron deficiency by performing an indoor iron-addition experiment. Undaria pinnatifida sporophyte growth, thallus color, and photosynthetic pigment content were all restored by the addition of dissolved iron solution, which the concentrations were between 10 and 50 μg/L. Iron addition made the sporophytes recover their lightness (L*) and redness (a*). In addition, the color was restored more quickly as the amount of added iron was increased. The discoloration of iron-deficient Undaria pinnatifida sporophytes can thus be restored by iron addition.