‘Huangguan No. 1’ female and male gametophyte clones were cultured in sterilized seawater with different concentrations of nitrogen (N) and phosphorus (P). The effects of different iron ion (Fe³⁺) concentrations on the growth and development of the gametophytes were studied via observation of the morphological changes, the maximum fluorescence yield of photosystem II (), and calculation of the gametogenesis rate (including the oogonium formation rate, ovulation rate, and seedling formation rate). The results showed that 3.60 μmol/L of Fe³⁺ was the most effective at promoting vegetative growth in gametophytes in seawater with N and P concentrations of 0.825 mmol/L and 0.0336 mmol/L, respectively. Under different N and P concentration conditions, the maximum fluorescence yield of kelp gametophyte could be increased when the Fe³⁺ concentration was 3.60-17.80 μmol/L, and there was no significant difference between each concentration group (>0.05). Under different nitrogen and phosphorus concentrations, the oogonium formation rate, ovulation rate, and seedling formation rate were significantly improved at Fe³⁺ concentrations of 3.60 μmol/L, 8.90 μmol/L, and 17.80 μmol/L. So the iron could increase the developmental process. When the nutrient concentration of N and P reached 0.275 mmol/L and 0.0112 mmol/L, 0.36-17.80 μmol/L Fe³⁺ could promote the gametophyte from vegetative growth to reproductive growth. At Fe³⁺ concentrations of 3.60-17.80 μmol/L, the gametophyte development indicators did not differ (>0.05) among different nitrogen and phosphorus groups. Even if the N and P concentrations reach 0.825 mmol/L and 0.0336 mmol/L, all of the gametes remained in vegetative growth without development under the iron-free conditions. If iron was sufficiently available, most of the gametes (65%) remained in vegetative growth under low nitrogen and phosphorus conditions. A small proportion of gametophytes (35%) entered the developmental state, but the development lagged behind the other N and P concentration groups in the experiment. These results indicated that iron, nitrogen, and phosphorus have synergistic effects on the growth and development of gametophytes and that iron is the key factor in the transition from vegetative growth to gametogenesis. Under appropriate concentrations of nitrogen (0.275 mmol/L) and phosphorus (0.0112 mmol/L), additional Fe³⁺ (3.60 μmol/L) had the greatest effect on the transition process. This study provided a theoretical basis for iron, nitrogen, and phosphorus use during the culture process of gametophytes seedlings and promotes the development of its seedling industry.