van der Waals (vdW) heterojunctions offer many routes for advanced interface engineering toward superior optoelectronic functionality. To this end, the combination of 2D transition metal dichalcogenides (TMDCs) with metal halide perovskites has shown great potential for applications in photovoltaics and photodetectors. The electronic energy level alignment at such heterojunctions, i.e., the relative alignment of valence and conduction bands of the two materials, is crucial for their functionality, but its experimental determination is notoriously challenging. In this contribution, we determine the energy level alignment for the vdW heterojunction composed of monolayer molybdenum disulfide (ML-MoS2) and a triple cation-mixed halide perovskite, enabled by surface cleaning by argon cluster sputtering. This effectively removes surface contaminants from the perovskite/ML-MoS2 stack without causing damage, enabling direct determination of the band alignment at the interface using ultraviolet and X-ray photoelectron spectroscopy. Our results reveal a type-II band alignment at the perovskite/ML-MoS2 interface. Importantly, the interfacial energy levels are not fixed once the heterojunction is formed, but the MoS2 energy levels shift relative to those of the perovskite under 1 sun illumination compared to the dark, by up to 0.25 eV. This energy level realignment, under conditions mimicking a photovoltaic device under operation, is attributed to photogenerated electron accumulation in the ML-MoS2. Microscopic photoluminescence (PL) measurements reveal significant quenching of the perovskite PL signal in the heterojunction, confirming efficient charge transfer and the establishment of a type-II heterojunction. These results demonstrate a “living” heterojunction energy landscape, opening up novel avenues for engineering perovskite/TMDCs vdW heterojunctions for optoelectronic devices.