Ti3C2Tx MXenes have been shown to be promising candidates for use in various applications. Herein, we prepare functionalized Ti3C2Tx MXene nanosheets doped with cesium (Cs) and introduce them into a lead iodide (PbI2) precursor solution for perovskite solar cells (PSCs) through a two-step deposition method, combining the advantages of both additives. Our theoretical and experimental analysis reveal that cesium plays an important role in improving perovskite crystallization and thus leads to enlarged crystal grains, long-lived carrier lifetimes, and reduced charge recombination compared with the devices fabricated without and with undoped Ti3C2Tx MXene. PSCs integrated with cesium-doped Ti3C2Tx MXene deliver high photovoltaic efficiency of up to 21.57% with excellent thermal stability. The incorporation of Cs-Ti3C2Tx paves the way to further improve the photovoltaic performance and thermal stability of PSCs by indirectly introducing dopants.