Corals have been found living in extreme environments such as the Persian/Arabian Gulf (PAG), the world’s warmest sea, represent a promising resource to study the phenotypic and genetic architecture of thermal tolerance in the coral holobiont. Here, we applied acute heat stress assays to coral populations in the southern PAG and neighbouring Gulf of Oman. We found that although southern PAG coral exhibit the highest absolute thermal tolerance threshold measured thus far, they already experience baseline stress under the contemporary climate, suggesting little cushion space to tolerate further warming. Data files provided: NOAA_CW_NetCDF4_extracted.zip: Satellite-derived climatology data from CoRTAD version 6 (1982-2022; Saha et al. 2018) for thermal stress anomaly frequency standard deviation (TSA_freq_stdev), sea surface temperature anomaly frequency standard deviation (SSTA_freq_stdev), weekly SST, and maximum monthly mean (MMM; only 1985-2012; NetCDF4 file) were extracted using ‘cwsample’ from the NOAA Coastwatch Data Analysis Tool. NOAA_CW_ERDDAP.zip: SST and degree heating weeks (DHW) extracted using the NOAA CoastWatch ERDDAP GRIDDAP interface for all collection sites. UW_colony_photos.zip: underwater photographs of sampled colonies taken as a size and morphology reference. Photo labels denote samplecollectiondate_region_site_species_colonynumber. CTD.zip: CTD data for each collection site. Inputfile_PAG_site_parameters.txt: summarised CTD data for plotting. General note: The colloquial site name of SI (Snoopy Island) is also referred to as AA (Al Aqah) in the main paper text.