Hyper-Kamiokande, a 187~kton fiducial volume water Cherenkov detector to be built in Japan, is the next generation of the Super-Kamiokande experiment. Its broad physics program includes nucleon decay, neutrinos from astronomical and human-made beam, with the main focus to determine the leptonic CP violation. To detect the weak Cherenkov light generated by neutrino interactions or proton decay, the primary photo-detector candidate are 20-inch PMTs. In order to enhance the Hyper-Kamiokande physics sensitivity, the use of a smaller amount of mPMTs to be integrated in the 20" PMT configuration, is considered. A multi-PMT Optical Module based on a pressure vessel instrumented with multiple small diameter photosensors, readout electronics and power, offers several advantages as increased granularity, reduced dark rate, weaker sensitivity to Earth’s magnetic field, improved timing resolution and directional information with an almost isotropic field of view. In the first part of this talk, we will briefly present the multi-PMT modules developed for Hyper- Kamiokande. We will then show their positive impact on the sensitivity of the Hyper-Kamiokande Intermediate Water Cherenkov detector, which motivated their choice as the primary photosensor candidate. We will finally conclude by presenting the impact of these modules on the physics of the Hyper-Kamiokande far detector.