Here, we report that incorporation of size-controlled CdSe quantum dots (QDs) into perovskite photovoltaic cells (PSCs) boosts their light harvesting capability. X-ray photoemission and optical absorption spectroscopy analyses also show that the electronic structure of CdSe QDs makes them efficient charge transfer mediators between perovskite and Spiro-MeOTAD layers. In addition, electrochemical impedance spectroscopy experiments demonstrate that QDs help to decrease charge transfer resistance at the interfaces. Additionally, time-correlated single photon counting measurements show that small (2 nm) QDs enhance visible light collection of PSCs in the short wavelength region via Forster resonance energy transfer while large (4 nm) QDs improve light collection of PSCs in the long wavelength region via enhanced light backscattering at the perovskite/QD interface. Moreover, the photocurrent density in the PSCs with QDs retained over 95% of the initial value in a 100 h stability test, thus supporting that the perovskite layer that has been encapsulated with QDs acts to prevent penetration of water molecules through the perovskite layer. Consequently, these results support that utilization of size-controlled hybrid QDs could open up a new route to realize high-performance PSCs even under humid conditions.