<p>Wildfires emit large quantities of brown carbon, a class of light-absorbing organic aerosols whose climate effects remain poorly constrained. Brown carbon displays a broad spectrum of absorptivity, ranging from weakly absorbing chromophores in the near-ultraviolet to strongly absorbing species that extend into the visible spectrum—yet its optical properties, global distribution and radiative influence remain largely uncertain. Here we assess the global prevalence and optical characteristics of dark brown carbon through an integrated analysis of aircraft measurements, ground-based observations and satellite retrievals. We show that this strongly absorbing aerosol is widespread in wildfire plumes around the world. Measured dark brown carbon mass absorption efficiencies range from 0.5 to 1.5 m² g⁻¹ at 500 nm, with absorption frequently comparable to—or even exceeding—that of black carbon. When these observationally constrained optical properties are incorporated into a global aerosol–climate model, we estimate a direct radiative effect of +0.097 W m⁻² (spanning +0.050 to +0.276 W m⁻²) attributable to wildfire-derived brown carbon, with the upper bound surpassing black carbon’s contribution and extending into mid- and high-latitude regions, including the Arctic. These results position dark brown carbon as a critical yet underrecognized contributor to wildfire radiative forcing, highlighting the need to account for its substantial warming influence in climate assessments.</p>