Despite its simple structure and low degree of electronic correlation, SrTiO_{3} (STO) features collective phenomena linked to charge transport and, ultimately, superconductivity, that are not yet fully explained. Thus, a better insight into the nature of the quasiparticles shaping the electronic and conduction properties of STO is needed. We studied the low-energy excitations of bulk STO and of the LaAlO_{3}/SrTiO_{3} two-dimensional electron gas (2DEG) by Ti L_{3} edge resonant inelastic x-ray scattering. In all samples, we find the hallmark of polarons in the form of intense dd+phonon excitations, and a decrease of the LO3-mode electron-phonon coupling when going from insulating to highly conducting STO single crystals and heterostructures. Both results are attributed to the dynamic screening of the large polaron self-induced polarization, showing that the low-temperature physics of STO and STO-based 2DEGs is dominated by large polaron quasiparticles.