Global ocean warming and marine heatwaves are of increasing concern in marine ecosystems. Seagrasses are particularly vulnerable to rising temperatures, yet the thermal tolerance of seagrass seeds and seedlings—often more sensitive to environmental fluctuations—remains largely unknown. This study examines the physiological and plant-scale responses of P. australis propagules to increasing temperatures, specifically identifying the optimum and maximum temperature for photosynthesis, growth and survival. Differences in thermal optima provide an opportunity for resilience building where seeds from populations with higher thermal optima could be prioritised in restoration programs. Seeds were collected across a broad latitudinal temperature gradient (~800 km) and at a local scale (~20 km) from three Western Australian regions with distinct environmental temperatures—warmer (Geraldton, Leeman), warm (Shoalwater Bay, Woodman Point), and cold (Princess Royal Harbour). Seedlings were exposed to five temperature treatments (21°C, 24°C, 27°C, 30°C, and 33°C) in a long-term mesocosm experiment lasting three months. Preliminary investigations suggest that there is up to 1°C difference in the thermal optima for growth at both broad and local scales and across different response variables.