Alloyed lead-free double perovskites display intense photoluminescence, are environmentally friendly, and their devices show long-term operation. Thanks to these properties, which make them excellent warm white-emitting materials, they have recently received great attention in lighting applications. An important factor to tune the optical properties of alloyed lead-free double perovskites is the presence of self-trapped excitons. Here, it is demonstrated that in leadfree double perovskites, the strong electron–phonon coupling plays a crucial role in the generation of self-trapped excitons. The strong electron–phonon coupling is confirmed by a large Huang–Rhys factor and by the presence of multiphonon transitions. In particular, sharp emission lines superimposed on the broad photoluminescence emission band of one of these samples (Cs2Ag0.6Na0.4InCl6 0.5%Bi) are observed; these are due to the strong coupling of longitudinal-optical phonons with excited electronic states caused by the tetragonally distorted AgCl6 octahedrons. Such a strong coupling of longitudinal-optical phonons to electrons can effectively modulate the photophysical properties of alloyed double perovskites, and its understanding is, thus, of paramount importance for the design of future optoelectronic devices.