Iron is an essential micronutrient for seaweed cultivation, and its limitation can constrain growth and photosynthesis performance. The present study was conducted to evaluate the effects of different iron concentrations on growth, photosynthesis and physiological responses in the red algae Pyropia yezoensis. Biomass of P. yezoensis was pre-cultured for one week in a nutrient medium containing only nitrate and phosphate to deplete internal iron storage. Subsequently, the biomass was cultivated for five weeks under several iron concentrations (0, 1, 5, 10, 20 and 50 μM). Specific growth rate (SGR), photosynthetic efficiency (Fv/Fm), pigment contents (Chlorophyll a, carotenoids, phycoerythrin and phycocyanin), total protein and tissue carbon and nitrogen contents were analyzed. In this study, a sequential decline was observed under iron deficiency condition (0 μM). Chl a and carotenoids decreased from the first week, followed by a reduction in photosynthetic efficiency (Fv/Fm) from the second week. Both the growth rate and total protein content markedly declined after the third week. In contrast, excessive iron concentration (50 μM) had thallus bleaching and significant growth inhibition from fourth week. Through these results, the iron availability plays a key determinant role in P. yezoensis, where limited iron sequentially restricts the regulation of photosynthetic pigments and growth, leading to reduced physiological performance. In contrast, excessive iron concentrations suppressed thallus growth and pigment stability, indicating that both deficiency and excess can negatively affect seaweed cultivation.