<p>Purpose: Chitosan enhances tolerance to salinity in rice. Apoplastic flow plays a crucial role in the accumulation of sodium (Na⁺) in rice under salinity. This study investigated the effects of exogenous chitosan on apoplastic flow and Na⁺ uptake in NaCl-treated rice seedlings. Methods: We employed an apoplastic tracer, trisodium salt of 8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS), in order to evaluate apoplastic flow in rice (<i>Oryza sativa</i> L., cv. Nipponbare) seedlings that were hydroponically grown in the solution containing NaCl (0 and 25 mM), and chitosan (0 mg L^− 1, 10 mg L^− 1, and 50 mg L^− 1). Results: Application of 25 mM NaCl significantly increased PTS uptake and Na⁺ content in shoots but did not affect K⁺ content, resulting in a lower K⁺/Na⁺ ratio although 25 mM NaCl did not affect the seedling growth. The application of chitosan suppressed Na⁺-enhanced PTS uptake and Na⁺ accumulation in shoots without affecting the K⁺ content, which led to a higher K⁺/Na⁺ ratio. Moreover, chitosan did not affect the reducing sugar content or electrical conductivity in the solution containing NaCl. Conclusions: These results suggest that application of chitosan suppressed Na⁺-enhanced apoplastic flow to reduce Na⁺ uptake in rice seedlings.</p>