Cyanobacterial mats form in intertidal and shallow subtidal regions of arid coastlines, persisting under extreme temperature and salinity conditions. They are under increasing pressure from coastal development (e.g. salt farms) and sea level rise, but environmental drivers and ecological functions are poorly understood, hindering effective management. This study assessed the effects of salinity variation on cyanobacterial mat photosynthesis using a combination of field collections and manipulative experiments (0, 30, 60, 90, 120 psu). Photosynthetic activity and productivity were assessed using chlorophyll fluorescence (Pulse Amplitude Modulated fluorometry, MINI-PAM-II/R) and oxygen evolution (chambers with FireSting O2 technology). In the field, cyanobacterial mats were photosynthetically active in salinities from 0 psu to 60 psu and temperatures ranging from 15°C-40°C. Experimental analysis over daily cycles showed activity from freshwater (0 PSU) up to 120 PSU, with declines later in the day under higher salinity. Chamber experiments also detected gross primary production across the range of salinities. This research highlights the extreme range of salinity for photosynthesis in these ecosystems providing critical information for ecological models. It enhances understanding of cyanobacterial mat ecosystem function and will contribute to more effective decision making for the management and conservation of these ecosystems.