Seagrasses in estuaries and coastal zones globally are subject to multiple stressors including high temperature, hypersalinity, hypoxia, sulfide (H2S) toxicity, algal blooms, and extreme climatic events, all likely to increase under climate change. Tropical seagrasses, such as Thalassia testudinum an Atlantic-Caribbean late-successional habitat-forming species, have a high thermal and salinity tolerance, but large-scale mortality events (>50 km2) occur once thresholds (>60 salinity; >33 oC) are crossed. We examined internal seagrass pO2 and H2S dynamics using microsensors in response to hypoxia, low irradiance and hypersalinity of T. testudinum in Florida Bay (USA), a subtropical estuary with recurrent die-off events, and in laboratory experiments. Hypersalinity and high temperature minimize the solubility of water column O2 and disrupt internal O2 pressurization of seagrass leading to extreme hypoxia at night. Hypoxia enhances the probability of H2S intrusion into shoot meristems leading to tissue necrosis and detachment of shoots, an indication of an active die-off in the field. Our results support the hypothesis that meristem hypoxia and H2S intrusion in Florida Bay, and likely globally, particularly in highly organic or carbonate environments where sequestration of sulfides by iron is low, are primarily driven by insufficient internal plant oxidation under low light or hypersalinity and/or low pO2 in the water column. This occurs even when high irradiance sustains supersaturation (30-50 kPa) of tissue pO2 during the day. Based on these experimental and field microsensor results, we suggest two management objectives to minimize hypoxia and H2S-induced seagrass mortality: (1) minimize hypersaline conditions and maximize light penetration to increase the efficiency of internal seagrass O2 pressurization that has an important positive feedback on water column and sediment oxidation, and (2) reduce nutrient loads, temperature, and hypersaline conditions to keep water column pO2 at a maximum and lower sediment-meristem O2 demand.