Developing efficient earth-abundant transition metal-based electrocatalysts for the hydrogen evolution reaction (HER) is crucial for hydrogen production at scale. This paper reports that the buried electrocatalytic interfaces between Ni-Fe sulfide (NiFeS) nanosheets and TiO2 conformal coatings (about 5 nm) achieved remarkable HER activity improvement, lowering the HER overpotential from 170 mV to 107 mV at 50 mA cm 2 in a base. Non-HER active, permeable TiO2 coatings grown by atomic layer deposition (ALD) achieved continuous fine-tuning of the electronic properties at the buried TiO2/ NiFeS interfaces, as a novel strategy and the main factor for electron accumulation at the interface. Core-level and valence band X-ray photoelectron spectroscopy (XPS) was used to investigate the TiO2 electronicstructure tuning effect on the charge-transfer energetics during the HER. Their alkaline HER mechanism was elucidated by supplementing characterizations of membrane permeation, Tafel slope, and synchrotron X-ray absorption spectroscopy, which verified that the buried TiO2/ NiFeS interfaces are electrocatalytically active. This study offers a general strategy for improving the charge-transfer kinetics of an electrocatalytic system by confining catalysis at a permeable solid-solid interface. The broad applicability of permeable and tunable coatings potentially accelerates the optimization of earthabundant catalysts to achieve high performance under operationally relevant conditions.