In the past decade, intensive studies on monolayer MoS₂-based phototransistors have been carried out to achieve further enhanced optoelectronic characteristics. However, the intrinsic optoelectronic characteristics of monolayer MoS₂ have still not been explored until now because of unintended interferences, such as multiple reflections of incident light originating from commonly used opaque substrates. This leads to overestimated photoresponsive characteristics inevitably due to the enhanced photogating and photoconductive effects. Here, we reveal the intrinsic photoresponsive characteristics of monolayer MoS₂, including its internal responsivity and quantum efficiency, in fully transparent monolayer MoS₂ phototransistors employing a van der Waals heterostructure. Interestingly, as opposed to the previous reports, the internal photoresponsive characteristics do not significantly depend on the wavelength of the incident light as long as the electron-hole pairs are generated in the same k-space. This study provides a deeper understanding of the photoresponsive characteristics of MoS₂ and lays the foundation for two-dimensional materials-based transparent phototransistors.