Saccharina japonica is an important economic alga in China, and the kelp cultivated in China is derived from wild populations in Japan. The environmental temperature in Chinese cultivation areas is higher than that of kelp’s native habitat. In particular, in the southern farming areas of China, high-temperature stress is one of the key factors contributing to yield reduction and diseases in kelp. Therefore, heat tolerance is an important breeding objective in developing superior kelp varieties. The high-temperature tolerance of kelp gametophytes is closely related to that of sporophytes. Evaluating the high-temperature tolerance of gametophytes and selecting heat-resistant ones for hybrid breeding are crucial for developing new kelp varieties with enhanced heat tolerance.However, at present there is no precise evaluation index system for kelp's high-temperature tolerance traits, and there is still a lack of systematic and comprehensive evaluation and comparison of high-temperature tolerance among different varieties and strains.In this study, 100 samples of kelp gametophytes collected from different years and regions were used, covering the major cultivation varieties and strains in China over the past 40 years. Their high-temperature tolerance was comprehensively evaluated using growth and photosynthetic physiological indicators, with the aim of establishing an evaluation index system for high-temperature tolerance in kelp gametophytes and screening for high-temperature tolerant gametophyte materials. By observing the relative growth rates of kelp gametophytes at 15°C, 25°C, 26°C, and 27°C, it was determined that their lethal temperature is 27°C, and 25°C was chosen as the high-temperature stress condition for kelp gametophytes.Then, the relative growth rate and photosynthetic parameters (Fv/Fm、Y(Ⅱ)、Jmax and PARsat) of the kelp gametophytes were measured under conditions of 15°C and 25°C, respectively.The results showed that under high-temperature stress at 25°C, both the growth and photosynthetic parameters of the kelp gametophytes exhibited a significant decline. Moreover, the coefficients of variation for these parameters increased before and after high-temperature stress, indicating that under high-temperature stress, the previously consistent performance among different strains began to diverge. The distribution of each parameter became more dispersed, and the differences between strains increased. Integrating relative growth rate and chlorophyll fluorescence parameters, the heat tolerance of kelp gametophytes spanning more than 40 years was evaluated and compared from multiple dimensions.The results revealed that after high-temperature stress, the kelp gametophytes from the most recent 20s exhibited the highest relative growth rate, with the smallest decrease compared to their growth rate at the optimal temperature. This may be attributed to the fact that the sea water temperature in the primary cultivation areas of farmed kelp in our country is higher than that in their natural distribution regions, which continuously enhances the high-temperature tolerance in the offspring of these cultured populations.After high-temperature stress, the kelp gametophyte materials from the 1980s experienced a larger decline in the photosynthetic parameters Fv/Fm, Y(II), and Jmax compared to those from other decades, indicating that high-temperature stress had the greatest impact on their photosynthesis. This may be because the gametophytes from the 1980s were stored under low-temperature and weak-light conditions for the longest period, making them relatively more sensitive to high-temperature stress. In recent years, the rapid increase in temperature and the pursuit of higher yields in production may have contributed to the enhanced heat tolerance observed in kelp gametophyte growth during the 2020s. However, changes in photosynthetic capacity may require a longer period of acclimation and accumulation. Overall, from the 1980s to the 2020s, the heat tolerance of kelp gametophytes has shown a gradual increasing trend. This study also comprehensively and systematically evaluated the differences in heat tolerance by comparing the growth and photosynthetic parameters of kelp gametophyte clones from different regions.Under high-temperature stress, the kelp gametophytes from southern China exhibited the highest relative growth rate and the smallest reduction in growth; moreover, their photosynthetic parameters Fv/Fm, Y(Ⅱ), and Jmax declined less compared to those from northern China, indicating that their photosynthesis was less affected by high-temperature stress.Overall, kelp gametophytes in southern China exhibit stronger heat tolerance than those in northern China, possibly because the selective pressure of the high-temperature environment on kelp sporophytes has been genetically transmitted to the gametophyte cells. Integrating growth and chlorophyll fluorescence parameters, a heat tolerance evaluation model for kelp gametophytes was established—referred to as the comprehensive heat tolerance evaluation value (H value). Based on the H value, 100 gametophyte samples were classified by heat tolerance type, and three heat-tolerant germplasm materials were selected. In summary, this study developed an evaluation model for high-temperature tolerance in kelp gametophytes and, for the first time, systematically evaluated and compared the high-temperature tolerance of kelp varieties and strains in China over the past 40 years. A collection of superior high-temperature tolerant germplasm materials was obtained, which is of great significance for kelp germplasm innovation and the breeding of superior varieties.