Abstract: To investigate the salt adaptation mechanisms of coastal phytoplankton, this study selected the euryhaline cyanobacterium Synechococcus sp. PCC 7002, which inhabits coastal waters, for salt stress research. Using a whole-genome mutant library of Synechococcus constructed in our laboratory by sequential deletions in 5000 bp increments, we performed a salt-stress screen and obtained a mutant strain Δ480 that exhibited a response to salt stress. Subsequently, by individually knocking out the genes contained in the deleted fragment, the pseudouridine synthase-encoding gene rsuA was identified as the key gene responsible for the salt-sensitive phenotype of the mutant. Physiological parameters of the knockout mutant (ΔrsuA), complementation strain (ΔrsuA::rsuA), and overexpression strain (OE-rsuA) were measured under standard salt concentration (0.3 mol/L NaCl) and high-salt concentration (1.2 mol/L NaCl). The results showed that under high salt stress, ΔrsuA exhibited significantly reduced growth rate, chlorophyll a content, and maximum photochemical efficiency of photosystem II (F_v/F_m) compared to the wild type. The phenotype of the complementation strain was restored to wild-type levels, confirming that the phenotype was caused by inactivation of this gene. Transcriptomic analysis revealed that under normal conditions, cells compensate for the functional defect in ribosome assembly resulting from rsuA deletion through upregulating the expression of ribosome-related genes. However, under salt stress conditions, the cellular demand for stress proteins is dramatically amplified. The ΔrsuA mutant, due to insufficient functional ribosomes, fails to express sufficient stress-related proteins promptly, rendering it unable to adapt to high-salt conditions. In conclusion, this study identifies rsuA as a crucial gene for the adaptation of Synechococcus sp. PCC 7002 to high-salt environments. Its deletion leads to impaired photosynthetic performance and growth inhibition under salt stress, potentially by affecting ribosome function and translational homeostasis. This research reveals, for the first time, the function of RsuA in the salt stress adaptation of cyanobacteria, providing new insights into the environmental adaptation mechanisms of cyanobacteria.