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We identify a sub-Neptune exoplanet (R-p = 2.5 +/- 0.2 R-circle plus) transiting a solar twin in the Ruprecht 147 star cluster (3 Gyr, 300 pc, [Fe/H] = +0.1 dex). The similar to 81 day light curve for EPIC 219800881 (V= 12.71) from K2 Campaign 7 shows six transits with a period of 13.84 days, a depth of similar to 0.06%, and a duration of similar to 4 hr. Based on our analysis of high-resolution MIKE spectra, broadband optical and NIR photometry, the cluster parallax and interstellar reddening, and isochrone models from PARSEC, Dartmouth, and MIST, we estimate the following properties for the host star: M-star = 1.01 +/- 0.03 M-circle dot, R-star = 0.95 +/- 0.03 R-circle dot, and T-eff = 5695 +/- 50 K. This star appears to be single based on our modeling of the photometry, the low radial velocity (RV) variability measured over nearly 10 yr, and Keck/NIRC2 adaptive optics imaging and aperture-masking interferometry. Applying a probabilistic mass-radius relation, we estimate that the mass of this planet is M-p = 7 + 5 - 3 M-circle plus, which would cause an RV semi-amplitude of K = 2 +/- 1 ms(-1) that may be measurable with existing precise RV facilities. After statistically validating this planet with BLENDER, we now designate it K2-231b, making it the second substellar object to be discovered in Ruprecht 147 and the first planet; it joins the small but growing ranks of 22 other planets and three candidates found in open clusters.