Interactions mediated by a surrounding medium appear across physics, from magnetic impurities in metals to fundamental particles mediated by gauge bosons. Such interactions is particularly enhanced in a heavy boson-light fermion mixture. In a series of experiments, we observe fermion-mediated interactions and scattering resonance, which connects the Ruderman-Kittel-Kasuya-Yosida mechanism, polaron interactions and Efimov physics in a degenerate quantum Bose-Fermi mixture.