Long-term stability is an essential requirement for perovskite solar cells to be commercially viable. Encapsulating 3D perovskites with 2D perovskite structures is an effective strategy for improving resistance to moisture. However, long-chain alkylammonium cation-based 2D perovskites have been rarely studied in solar cells. Here, we study three different alkyl chain length organic cation-based 2D perovskite coatings for 3D perovskites. The 2D perovskite incorporated solar cells show significant improvement in solar cell stability with limited compromise in solar cell efficiency, with the longest alkyl chain length sample showing only a 20% drop in power conversion efficiency after 6 months at a relative humidity of 25-80%, and could be completely immersed in water for a few minutes before degradation started. The 2D perovskite coating also mitigated non-radiative recombination in the light-absorbing 3D perovskite, leading to an enhancement in the open circuit voltage. These findings suggest that long-chain alkylammonium cation based 2D perovskites can improve the environmental stability of 3D based perovskites without significant losses to device performance. Moisture resistance is vital for commercializing perovskite solar cells. Here, long-chain alkylammonium cation-based 2D perovskites are used to coat 3D perovskite, enabling stable performance for six months with only a 20 % drop in power conversion efficiency.