Electrical characterisation of perovskite solar cells consisting of room‐temperature atomic‐layer‐deposited aluminium oxide (RT‐ALD‐Al₂O₃) film on top of a methyl ammonium lead triiodide (CH₃NH₃PbI₃) absorber showed excellent stability of the power conversion efficiency (PCE) over a long time. Under the same environmental conditions (for 355 d), the average PCE of solar cells without the ALD layer decreased from 13.6 to 9.6 %, whereas that of solar cells containing 9 ALD cycles of depositing RT‐ALD‐Al₂O₃ on top of CH₃NH₃PbI₃ increased from 9.4 to 10.8 %. Spectromicroscopic investigations of the ALD/perovskite interface revealed that the maximum PCE with the ALD layer is obtained when the so‐called perovskite cleaning process induced by ALD precursors is complete. The PCE enhancement over time is probably related to a self‐healing process induced by the RT‐ALD‐Al₂O₃ film. This work may provide a new direction for further improving the long‐term stability and performance of perovskite solar cells.