Narrowing the bandgap of lead-free double-perovskite Cs₂AgBiBr₆ is required for using this material in future photovoltaics. Herein, we demonstrate a bandgap engineering of Cs₂AgBiBr₆ by introducing Sb to substitute up to 75% of Bi via a versatile solution-processed method in dimethyl sulfoxide at 180 °C. The resultant Cs₂AgSb_xBi_1−xBr₆ (x = 0, 0.25, 0.50, 0.75) thin films possess high crystallinity and good thermostability. Moreover, the Sb substitution enables an obvious bandgap reduction of 0.25 eV. The fabricated solar cell using the Cs₂AgSb_xBi_1−xBr₆ (x = 0.25) thin film obtained an increased performance than the reference Cs₂AgBiBr₆. The effective bandgap narrowing via a facile solution method might accelerate the development of Cs₂AgBiBr₆-based materials for photovoltaic applications.