The association between long-term exposure to ambient air pollutants and severe COVID-19 is uncertain. We followed 4,660,502 adults from the general population in 2020 in Catalonia, Spain. Cox proportional models were fit to evaluate the association between annual averages of PM2.5, NO2, BC, and O3at each participant’s residential address and severe COVID-19. Higher exposure to PM2.5,NO2,and BC was associated with an increased risk of COVID-19 hospitalization, ICU admission, death, and hospital length of stay. An increase of 3.2 µg/m3of PM2.5was associated with a 19% (95% CI, 16–21) increase in hospitalizations. An increase of 16.1 µg/m3of NO2was associated with a 42% (95% CI, 30–55) increase in ICU admissions. An increase of 0.7 µg/m3of BC was associated with a 6% (95% CI, 0–13) increase in deaths. O3was positively associated with severe outcomes when adjusted by NO2. Our study contributes robust evidence that long-term exposure to air pollutants is associated with severe COVID-19.