Exposure to outdoor fine particulate matter (PM2.5) is a leading risk factor for mortality. We develop global estimates of annual PM2.5 concentrations and trends for 1998–2018 using advances in satellite observations, chemical transport modeling, and ground-based monitoring. Aerosol optical depths (AODs) from advanced satellite products including finer resolution, increased global coverage, and improved long-term stability are combined and related to surface PM2.5 concentrations using geophysical relationships between surface PM2.5 and AOD simulated by the GEOS-Chem chemical transport model with updated algorithms. The resultant annual mean geophysical PM2.5 estimates are highly consistent with globally distributed ground monitors (R2 = 0.81; slope = 0.90). Geographically weighted regression is applied to the geophysical PM2.5 estimates to predict and account for the residual bias with PM2.5 monitors, yielding even higher cross validated agreement (R2 = 0.90–0.92; slope = 0.90–0.97) with ground monitors and improved agreement compared to all earlier global estimates. The consistent long-term satellite AOD and simulation enable trend assessment over a 21 year period, identifying significant trends for eastern North America (?0.28 ± 0.03 ?g/m3/yr), Europe (?0.15 ± 0.03 ?g/m3/yr), India (1.13 ± 0.15 ?g/m3/yr), and globally (0.04 ± 0.02 ?g/m3/yr). The positive trend (2.44 ± 0.44 ?g/m3/yr) for India over 2005–2013 and the negative trend (?3.37 ± 0.38 ?g/m3/yr) for China over 2011–2018 are remarkable, with implications for the health of billions of people.