We report the discovery of EPIC 219388192 b, a transiting brown dwarf in a 5.3-day orbit around a member star of Ruprecht-147, the oldest nearby open cluster association, which was photometrically monitored by K2 during its Campaign 7. We combine the K2 time-series data with ground-based adaptive optics imaging and high resolution spectroscopy to rule out false positive scenarios and determine the main parameters of the system. EPIC 219388192 b has a radius of $R_mathrm{b}$=$0.937pm0.042$~$mathrm{R_{Jup}}$ and mass of $M_mathrm{b}$=$36.50pm0.09$~$mathrm{M_{Jup}}$, yielding a mean density of $59.0pm8.1$~$mathrm{g,cm^{-3}}$. The host star is nearly a Solar twin with mass $M_star$=$0.99pm0.05$~$mathrm{M_{odot}}$, radius $R_star$=$1.01pm0.04$~$mathrm{R_{odot}}$, effective temperature $mathrm{T_{eff}}$=$5850pm85$~K and iron abundance [Fe/H]=$0.03pm0.08$~dex. Its age, spectroscopic distance, and reddening are consistent with those of Ruprecht-147, corroborating its cluster membership. EPIC 219388192 b is the first brown dwarf with precise determinations of mass, radius and age, and serves as benchmark for evolutionary models in the sub-stellar regime.
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