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Abstract The long‐term operational stability of perovskite solar cells (PSCs) remains a key challenge impeding their commercialization, particularly due to ambient environments (e.g., moisture, oxygen, heat)‐induced degradation. Carbon electrode‐based PSCs have emerged as cost‐effective and relatively stable alternatives to metal electrode‐based devices due to carbon materials' hydrophobic behavior, yet they still lag in both long‐term durability and power conversion efficiency (PCE). In this work, an ultrathin hydrophobic ligand‐modified core–shell Cd(S,Se)/ZnS quantum dots (QDs) capping layer is introduced as a multifunctional interfacial modifier for carbon‐electrode‐based PSCs. This oleic acid ligand‐modified QDs capping layer exhibits inherent hydrophobicity, effectively serving as a moisture barrier to retard perovskite degradation under ambient conditions. Furthermore, the strong interfacial bonding between the QDs and perovskite halide surfaces leads to efficient trap state passivation, reducing trap density and creating a more uniform electrical contact. The modified QDs/perovskite interface also features an elevated conduction band edge, promoting improved charge extraction. As a result, devices incorporating this quantum dot capping layer retain 98% of their initial PCE after 450 h of ambient aging and achieve a champion efficiency of 20.74%. This strategy highlights the potential of hydrophobic ligand‐modified chalcogenide QDs as surface modifiers to enhance both the stability and performance of carbon‐based PSCs, offering a promising route toward scalable fabrication of durable perovskite solar modules.