Abstract: The thalli of the economically important red alga Bangia fuscopurpurea mainly inhabit the intertidal rocks. Consequently, they experience approximately two dehydration/rehydration cycles per day. During the tidal cycle, the thalli can lose around 90% of water at low tide and rapidly recover to normal water content after rehydration, indicating that B. fuscopurpurea has the ability to adapt to the rapid water changes induced by the tidal cycle. However, litter is known about the mechanisms by which B. fuscopurpurea adapts to tidal changes. Therefore, a wild strain of B. fuscopurpurea was used to assess its response to 2h dehydration (with 5 water loss rates: 0, 30%, 60%, 90%) and 2h rehydration (pretreated with 2h dehydration at a 90% water loss rate) by examining physiological traits and the transcriptome. The present study found that the maximum photochemical quantum yield (F_v/F_m) of B. fuscopurpurea decreased significantly with the increasing dehydration level. Meanwhile, the contents of superoxide anion (\rmO^-_2 ) and hydrogen peroxide (H₂O₂) increased significantly and peaked at 30% and 60% water loss, respectively. The content of malondialdehyde (MDA) also increased with dehydration level. After rehydration, F_v/F_m and contents of \rmO^-_2 H₂O₂, and MDA returned to the levels of 0 water loss rate (WL0%) treatment group. Activities of superoxide dismutase (SOD) showed a decreasing trend with increasing desiccation. Ascorbate peroxidase (APX) and peroxidase (POD) increased significantly by dehydration, both peaking at 60% water loss, while catalase (CAT) did not change significantly. After rehydration, the activities of SOD, CAT, APX, and POD all returned to the levels of the WL 0% treatment. Dehydration significantly elevated proline and soluble sugar contents of B. fuscopurpurea. Additionally, the contents of phycoerythrin and allophycocyanin increased markedly at 60% water loss. These findings suggest that the algae’s adaptation to dehydration is facilitated by enhanced intracellular antioxidant enzyme activities and antioxidant contents, and by reducing photosynthesis to minimize cell damage. In addition, by comparing the transcriptional expression profiles of B. fuscopurpurea under different dehydration and rehydration treatmetns, we screened 3043 differential genes. Differential gene enrichment analysis revealed that genes related to ribosomal proteins and heat-excited proteins were up-regulated by dehydration treatment, while genes related to amino acid synthesis, aminoacyl-tRNA synthetase (aaRS), and ribosomal biogenesis-mediated ribosomal protein-related genes were significantly down-regulated by rehydration. This study further analyzed the expression patterns of these key genes and found that algae maintain intracellular homeostasis by synthesizing dehydration-tolerant ribosomal proteins and initiating defense functions such as molecular chaperones. In response to rehydration, the inhibition of amino acid metabolism, aaRS, and ribosome biogenesis-mediated synthesis of ribosomal proteins was employed to conserve energy. In addition, efforts were made to enhance the energy supply from the light reactions of photosynthesis and the transport of toxic metabolites to assist B. fuscopurpurea in resuming metabolic activities. In summary, this study analyzed the physiological and transcriptional responses of B. fuscopurpurea to dehydration and rehydration, and identified key response genes. The results can contribute to a deeper understanding of the response of intertidal macroalgae to rapid water changes, as well as provide a data reference for the selection and breeding of B. fuscopurpurea.