Interlayer interactions in a heterostructure offer a powerful method to realize exotic quantum phenomena. The physics of interlayer interactions has received renewed momentum with the emergence of 2D materials where the reduced electrical screening provides an ideal platform to study many-body interactions of their quasiparticles. In this letter, we report the observation of a new quasiparticle, the ‘polaronic trion’, resulting from a strong interfacial coupling of MoS2 directly grown on SrTiO3 (STO), where the charged quasiparticle, trion in MoS2 is dressed by the time-dependent dipolar electric field of a soft phonon mode in STO. We observe from temperature dependent photoluminescence emission that the binding energy of this quasiparticle can be tuned up to 90meV by varying the trion-phonon coupling as a function of temperature, electric field and substrate orientation. Our work provides a platform to realize unprecedented selective tunability of the quasiparticles in 2D semiconductors, driven by many body effects.