In the context of perovskite solar cells (PSCs), enhancing device performance often involves adding a small excess of lead iodide (PbI2) to the precursor solution. However, the presence of unreacted PbI2 can lead to accelerated degradation compromising long-term stability. This study addresses this issue through supramolecular complex engineering by introducing beta-cyclodextrin (beta-CD) into a triple cation perovskite to effectively prevent the crystallization of residual PbI2. This approach results in uniform crystal growth and the passivation of undercoordinated lead cation defects. The use of beta-CD leads to a PSC with an improved power conversion efficiency (PCE) of 21.36%, surpassing the control, and enhanced stability against aggressive thermal stress and high humidity (85% RH). This is supported by optical and morphological investigations, underscoring the role of beta-CD to maintain the desired perovskite phase. Notably, in comparison to the beta-CD-free control, the beta-CD-treated sample exhibited minimal bandgap shifts of 3 meV after 1170 hours of moisture exposure. Furthermore, this method not only passivates unreacted PbI2 but also provides valuable insights into the role of beta-CD in hybrid perovskite solar cells. Additional tests with maltose as a non-cyclic control were conducted and confirm the superior ability of beta-CD to enhance perovskite film stability under harsh conditions. The formation of a supramolecular system between beta-CD and perovskite holds promise as a strategy to control perovskite precursor chemistry, material structure, and subsequent device performance and stability.|By incorporating beta-cyclodextrin, mitigation of residual PbI2 crystallization, control of perovskite chemistry, and uniform crystal growth, leading to improved solar cell performance and stability were demonstrated.