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We investigated the photosynthetic characteristics of the crustose coralline alga <i>Pneophyllum fragile</i> (Corallinales, Rhodophyta) according to elevated water temperature and irradiance on the coast of Jeju in 2018. <i>P</i>. <i>fragile</i> was cultured under different temperature (11 °C, 21 °C, 26 °C, and 31 °C) and irradiance (0–1250 μmol photon m⁻² s⁻¹) conditions. Oxygen (O₂) concentrations at the <i>P</i>. <i>fragile</i> mat–water interface (MWI) were measured using an O₂ microsensor. At the MWI, the diffusive boundary layer thicknesses ranged from 200 to 400 μm. The O₂ concentrations at the mat surface increased in response to increasing irradiance, and reached 344% air saturation. The maximum photosynthesis capacity (P_max) and respiration rate in the dark (R_d) at 31 °C were about 3 times higher than those recorded at 11 °C. The compensation irradiance (E_c) and saturation irradiance (E_k) increased with increasing water temperature. The P_max, R_d, and E_c were statistically correlated with temperature (<i>p</i> < 0.05). The E_k increased up to 833 μmol photon m⁻² s⁻¹ at 31 °C and exhibited a strong dependence on irradiance at high temperatures. The adaptability of <i>P</i>. <i>fragile</i> to high temperatures and strong irradiance was distinct from that observed for coralline algae in other temperate waters.