We report the observation of efficient and low-noise frequency conversion between two microwave modes, mediated by the motion of a mechanical resonator subjected to radiation pressure. We achieve coherent conversion of more than $10^{12}~mathrm{photons/s}$ with a $95mathrm{%}$ efficiency and a $14~mathrm{kHz}$ bandwidth. With less than $10^{-1}~mathrm{photons cdot s^{-1}cdot Hz^{-1}}$ of added noise, this optomechanical frequency converter is suitable for quantum state transduction. We show the ability to operate this converter as a tunable beam splitter, with direct applications for photon routing and communication through complex quantum networks.
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