The photoluminescence properties of the phosphor (Sr,Mg)_3(PO_4)_2:Sn^2+ were investigated for luminescence thermometry in fluids. The luminescence emission intensity at 300 K is on the order of 10^6 photons per particle and the lifetime is 26 μs. With increasing temperature, the wide emission band exhibits a pronounced blue shift, which can be exploited for temperature imaging using a two-colour ratiometric approach. The measured temperature sensitivity in an imaging configuration is 0.6%/K at 300 K. The T_50 quenching temperature is 650 K, similar to the phosphor BaMgAl_10O_17:Eu^2+, but the estimated temperature sensitivity at 700 K is a factor 7 higher (0.36%/K). Moreover, the excitation laser fluence has a negligible effect on the measured temperature (0.6 K for a 10% change in the fluence), a five-to-tenfold improvement over phosphors previously investigated for fluid thermometry. The phosphor (Sr,Mg)_3(PO_4)_2:Sn^2+ therefore offers significant improvements for thermometry applications in turbulent flows in the 300–900 K range.