Context.The astrophysical characterisation of sources is among the major new data products in the thirdGaiaData Release (DR3). In particular, there are stellar parameters for 471 million sources estimated from low-resolution BP/RP spectra.Aims.We present the General Stellar Parameterizer from Photometry (GSP-Phot), which is part of the astrophysical parameters inference system (Apsis). GSP-Phot is designed to produce a homogeneous catalogue of parameters for hundreds of millions of single non-variable stars based on their astrometry, photometry, and low-resolution BP/RP spectra. These parameters are effective temperature, surface gravity, metallicity, absoluteM_Gmagnitude, radius, distance, and extinction for each star.Methods.GSP-Phot uses a Bayesian forward-modelling approach to simultaneously fit the BP/RP spectrum, parallax, and apparentGmagnitude. A major design feature of GSP-Phot is the use of the apparent flux levels of BP/RP spectra to derive, in combination with isochrone models, tight observational constraints on radii and distances. We carefully validate the uncertainty estimates by exploiting repeatGaiaobservations of the same source.Results.The data release includes GSP-Phot results for 471 million sources withG < 19. Typical differences to literature values are 110 K forT_effand 0.2–0.25 for log g, but these depend strongly on data quality. In particular, GSP-Phot results are significantly better for stars with good parallax measurements (ϖ/σ_ϖ > 20), mostly within 2 kpc. Metallicity estimates exhibit substantial biases compared to literature values and are only useful at a qualitative level. However, we provide an empirical calibration of our metallicity estimates that largely removes these biases. ExtinctionsA₀andA_BPshow typical differences from reference values of 0.07–0.09 mag. MCMC samples of the parameters are also available for 95% of the sources.Conclusions.GSP-Phot provides a homogeneous catalogue of stellar parameters, distances, and extinctions that can be used for various purposes, such as sample selections (OB stars, red giants, solar analogues etc.). In the context of asteroseismology or ground-based interferometry, where targets are usually bright and have good parallax measurements, GSP-Phot results should be particularly useful for combined analysis or target selection.