Formamidinium tin triiodide (FASnI3) is a strong contender for sustainable harvesting of solar energy and further optoelectronic applications. So far, only a few studies have considered its fundamental structure–property relationships, given the challenge of ensuring a high material quality. In a concerted effort, we here study high-quality FASnI3 single crystals through a combination of X-ray crystallography, density-functional-theory-based electronic structure calculations, and photoluminescence spectroscopy from room temperature down to 4 K. The luminescence exhibits irregular trends upon cooling with a generally strong intensity increase, but a range of negative thermal quenching, leading to an intensity maximum around 185 K which is absent in low-quality samples. Differences in the photoluminescence peak position and density-functional-theory-calculated band-gap energies highlight the importance of dynamic processes to the observable properties of FASnI3. The presented data offer deeper insight into the temperature-dependent characteristics of this halide perovskite and present opportunities for future exploration of its optoelectronic properties.