Abstract: Seasonal variations in seawater temperature constitute a key environmental factor regulating the growth and development of Betaphycus gelatinus, which frequently encounters both low- and high-temperature stress during its cultivation. Arginine functions as a metabolic precursor for the synthesis of nitric oxide and polyamines, and plays a crucial regulatory role in plant thermotolerance. Nevertheless, it remains uncertain whether exogenous arginine application can enhance the temperature stress tolerance of B. gelatinus. To investigate the effects of low- and high-temperature stress on the physiological activity of B. gelatinus and to evaluate the alleviating role of exogenous L-arginine under temperature stress, B. gelatinus thalli were exposed for 2 h to low temperature (15 ℃, LT), optimum temperature (27 ℃, MT), and high temperature (36 ℃, HT). Thalli without L-arginine addition served as the control (CK), while treatments with 6.25 mmol/L (LC) and 12.5 mmol/L (HC) L-arginine were used as experimental groups. Photosynthetic activity, pigment contents, antioxidant enzyme activities, and related indices were measured to assess the mitigating effects of L-arginine on temperature stress in B. gelatinus. The results showed that photosynthetic activity of B. gelatinus decreased significantly under both low- and high-temperature stress. The LC treatment significantly alleviated the inhibitory effects of low and high temperatures on photosynthetic activity, whereas the HC treatment only alleviated the inhibition under high-temperature stress. Chlorophyll a content remained unchanged in both the CK-LT and CK-HT groups, while carotenoid content increased significantly, with no significant difference in carotenoid content between the LC and HC groups. Under low temperature, the contents of phycoerythrin and phycocyanin decreased significantly; however, there was no significant difference in phycoerythrin (PE) and phycocyanin (PC) contents between the LC-LT group and the LC-MT group. Low and high temperatures had significant effects on ascorbate peroxidase (APX) and catalase (CAT) activities, and these effects were markedly reduced after L-arginine addition. Glutathione content decreased significantly under low temperature but recovered after L-arginine addition. Exogenous L-arginine also significantly reduced malondialdehyde (MDA) accumulation under stress conditions. These findings indicate that supplementation with 6.25 mmol/L L-arginine helps B. gelatinus cope with temperature stress. The results provide a theoretical basis for the application of arginine to alleviate temperature stress in B. gelatinus.