Emerging perovskite semiconductors have received considerable attention for transistor devices. However, due to the limited chemical stability of perovskites, the gate dielectric layer and its interface remain unexplored. Here, we propose an interfacial functionalization with a conjugated polymer to enable high-capacitance electrolyte dielectric deposition in top-gate perovskite transistors. This multifunctional approach, exploiting orthogonal solution-processed conjugated polymers allows the blocking of chemical diffusion during the deposition of the dielectric, allows passivation of defects on the perovskite surface, resulting in air stability enhancement, and boosts mobility via the formation of perovskite-polymer hybrid channels. The optimized conjugated polymer-capped lead iodide perovskite transistors showed remarkable hole mobility of over 30 and 15 cm2 V–1 s–1 on rigid and flexible substrates, respectively.