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We present the discovery of an optical transient (OT) in Messier 51,designated M51 OT2019-1 (also ZTF19aadyppr, AT 2019abn, ATLAS19bzl), by theZwicky Transient Facility (ZTF). The OT rose over 15 days to an observedluminosity of $M_r=-13$ (${\nu}L_{\nu}=9\times10^6~L_{\odot}$), in theluminosity gap between novae and typical supernovae (SNe). Spectra during theoutburst show a red continuum, Balmer emission with a velocity width of$\approx400$ km s$^{-1}$, Ca II and [Ca II] emission, and absorption featurescharacteristic of an F-type supergiant. The spectra and multiband light curvesare similar to the so-called "SN impostors" and intermediate-luminosity redtransients (ILRTs). We directly identify the likely progenitor in archivalSpitzer Space Telescope imaging with a $4.5~\mu$m luminosity of$M_{[4.5]}\approx-12.2$ and a $[3.6]-[4.5]$ color redder than 0.74 mag, similarto those of the prototype ILRTs SN 2008S and NGC 300 OT2008-1. Intensivemonitoring of M51 with Spitzer further reveals evidence for variability of theprogenitor candidate at [4.5] in the years before the OT. The progenitor is notdetected in pre-outburst Hubble Space Telescope optical and near-IR images. Theoptical colors during outburst combined with spectroscopic temperatureconstraints imply a higher reddening of $E(B-V)\approx0.7$ mag and higherintrinsic luminosity of $M_r\approx-14.9$(${\nu}L_{\nu}=5.3\times10^7~L_{\odot}$) near peak than seen in previous ILRTcandidates. Moreover, the extinction estimate is higher on the rise than on theplateau, suggestive of an extended phase of circumstellar dust destruction.These results, enabled by the early discovery of M51 OT2019-1 and extensivepre-outburst archival coverage, offer new clues about the debated origins ofILRTs and may challenge the hypothesis that they arise from theelectron-capture induced collapse of extreme asymptotic giant branch stars.

Discovery of an intermediate-luminosity red transient in M51 and its likely dust-obscured, infrared-variable progenitor