Abstract In this work, we employ atomic layer deposition (ALD) to form Al2O3 layer as an encapsulant for perovskite solar cells (PSCs). Al2O3 layer deposited at temperature as low as 95 °C achieves water vapor transmission rate (WVTR) of 1.84 × 10−2 g m−2 d−1 at 45 °C–100%RH when thermal ALD is used. In order to test the moisture barrier capability of Al2O3 layer for PSCs, mesoporous perovskite devices, with spiro-OMeTAD or PTAA as hole transport layer (HTM) encapsulated by 50 nm Al2O3 film, are exposed to 65 °C–85%RH for 350 h and their stabilities are monitored. We find that the color of perovskite does not change after 350 h of exposure regardless of the type of HTM used. With regards to Th-ALD encapsulated devices, PTAA based PSCs experienced a smaller power conversion efficiency (PCE) drop than spiro-OMeTAD based PSCs after thermal stress at 65 °C. This is due to the presence of pinholes within spiro-OMeTAD layer after thermal stress which are not observed in PTAA. Finally, we successfully achieve excellent durability test results for mesoporous (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15/PTAA devices encapsulated by 50 nm Al2O3 with less than 4% drop in PCE after 7500 h (> 10 months) of exposure to 50%RH under room temperature.