We present the second multi-frequency radio detection of a reverse shock in a γ-ray burst. By combining our extensive radio observations of the Fermi -LAT GRB 160509A at z = 1.17 up to 20 days after the burst with Swift X-ray observations and ground-based optical and near-infrared data, we show that the afterglow emission comprises distinct reverse shock and forward shock contributions: the reverse shock emission dominates in the radio band at 10 days, while the forward shock emission dominates in the X-ray, optical, and near-infrared bands. Through multi-wavelength modeling, we determine a circumburst density of n 0 ≈ 10 −3 cm −3 , supporting our previous suggestion that a low-density circumburst environment is conducive to the production of long-lasting reverse shock radiation in the radio band. We infer the presence of a large excess X-ray absorption column, N H ≈ 1.5 × 10 22 cm −2 , and a high rest-frame optical extinction, A V ≈ 3.4 mag. We identify a jet break in the X-ray light curve at t jet ≈ 6 d, and thus derive a jet opening angle of θ jet ≈ 4• , yielding a beamingcorrected kinetic energy and radiated γ-ray energy of E K ≈ 4 × 10 50 erg and E γ ≈ 1.3 × 10 51 erg (1-10 4 keV, rest frame), respectively. Consistency arguments connecting the forward and reverse shocks suggest a deceleration time of t dec ≈ 460 s ≈ T 90 , a Lorentz factor of Γ(t dec ) ≈ 330, and a reverse shock to forward shock fractional magnetic energy density ratio of R B ≡ B,RS / B,FS ≈ 8.