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Tropical seagrasses are at their highest risk of exposure to photosystem II (PSII) herbicides when elevated rainfall and runoff from farms transports these toxicants into coastal habitats during summer, coinciding with periods of elevated temperature. PSII herbicides, such as diuron, can increase the sensitivity of corals to thermal stress, but little is known of the potential for herbicides to impact the thermal optima of tropical seagrass. Here we employed a well-plate approach to experimentally assess the effects of diuron on the photosynthetic performance of Halophila ovalis leaves across a 25° C temperature range (36 combinations of these stressors across 15–40° C). The thermal optimum for photosynthetic efficiency (▵) in H. ovalis was 31° C while lower and higher temperatures reduced▵ as did all elevated concentrations of diuron. There were significant interactions between the effects of temperature and diuron …