Hypersalinity is a major stressor to seagrasses, particularly in highly evaporative coastal estuaries and lagoons as well as those subjected to brine effluent from desalination systems, a condition likely to be heightened under a warming climate. While hypersalinity has been well established to cause physiological dysfunction, the effects on internal seagrass pO₂ and O₂ flux to the water column has not been explicitly studied. We examined the pO₂ across the leaf diffusive boundary layer and estimated O₂ flux rates from T. testudinum leaves at 35, 45, 55, and 65 salinity in the light and dark using O₂ microsensors. Leaf segments were used to examine photophysiological responses and leaves of shoots in intact cores with multiple shoots to evaluate internal leaf pO₂ flux that incorporates whole-plant sinks. Steady-state pO₂ levels internally and at the leaf surface were also quantified at light saturation and in the dark. We found hypersalinity (45–65) primarily affected the O₂ dynamics of T. testudinum leaves in the light based on leaf segments and whole plants. Net and gross photosynthetic rates of leaf segments were significantly lower at 55 and 65 salinities compared to controls (35), and net photosynthesis at 65 salinity was significantly lower than at 55 salinity. In contrast, the light required to compensate for leaf respiration (I_comp) was not affected by hypersalinity, and photosynthetic efficiency was only significant between 35 and 55 salinities above compensation irradiances (Φ_{(I comp – 200)}), suggesting lesser effects of hypersalinity on photosynthesis at low irradiance below compensation (0–50 μmol photon m⁻² s⁻¹). We propose that hypersalinity, particularly ≥55 causes photosynthetic dysfunction limiting leaf O₂ production, specifically P_max at high irradiance. Further, our data indicate that with increasing hypersalinity more of the O₂ produced by leaves in intact shoots is consumed by meristems and/or belowground tissues and the sediment. These O₂ sinks are likely more significant than O₂ lost to leaf respiration (R_D) that was not shown to increase with salinity in experiments using leaves in isolation. Thus, hypersalinity not only affects the physiology, growth and reproduction of seagrasses, but also their O₂ balance. For seagrass species growing at their upper salinity thresholds, a management strategy that lowers hypersalinity exposure would limit loss of seagrasses to O₂ imbalance from hypersalinity effects on photosynthesis and ecosystem O₂ demand.