Abstract: To investigate the role of gametophyte predevelopment technology in the production of Saccharina japonica sporophyte seedlings, the gametophyte of S. japonicum was predeveloped under conditions of (13±1) ℃ temperature, 37.5 μmol·m⁻²s⁻¹ light intensity, and 10.0 g·L⁻¹ density. These predeveloped gametophytes were used to produce sporophyte seedlings in two 10-ton breeding ponds. After 11 days of predevelopment, the rates of seminal vesicle formation, ovulation, and sporophyte formation reached (98.1±3.3)%, (1.8±0.7)%, and (1.2±0.5)%, respectively. Compared to normal sporophyte seedlings, predeveloped ones reached commercial specifications 9.3% earlier in the 10-ton breeding ponds. Additionally, the seedling density with a body length exceeding 0.2 cm increased by 28.4%, while the seedling shedding rate decreased by 18.6%, and the seedling deficiency rate decreased by 70.8%. These findings indicate that gametophyte predevelopment technology can reduce cultivation time and improve the quality of sporophyte seedlings in breeding tanks.