Laser-assisted ferroelectric polarization switching recently has been proved to be an effective mean to manipulate the ferroelectric domain structure, but with the possibility to damage the specimen surface due to high energy input and large thermal expansion. Compared to laser, sunlight with moderate energy is expected to be more accessible. Here, we employed a simulated sunlight illumination instead of high-energy lasers to realize the sunlight-assisted ferroelectric domain switching in Sn₂P₂S₆ single crystals. The origin is the enhancement of localized carrier concentration due to the disproportionation reaction of Sn ions, which induces an additional internal field and assists the domain switching. The migration and accumulation of the Sn ions are also verified with scanning probe technique, which can be utilized as a resistive memory prototype. It is noteworthy that this memory effect can be significantly enhanced by sunlight illumination and, thus, make it suitable for the sunlight control of ferroelectric domain switching and ionic memory devices.