Metal halide perovskites are known to suffer from rapid degradation, limiting their direct applicability. Here, the degradation of phenethylammonium lead iodide (PEA2PbI4) two-dimensional perovskites under ambient conditions was studied using fluorescence, absorbance, and fluorescence lifetime measurements. It was demonstrated that the long-term stability of two-dimensional perovskites could be achieved through the encapsulation with hexagonal boron nitride. While un-encapsulated perovskite flakes degraded within hours, the encapsulated perovskites were stable for at least three months. In addition, encapsulation considerably improved the stability under laser irradiation. The environmental stability, combined with the improved durability under illumination, is a critical ingredient for thorough spectroscopic studies of the intrinsic optoelectronic properties of this material platform.