  Exoplanet atmosphere studies are often enriched by synergies with brown dwarf
analogs. However, many key molecules commonly seen in brown dwarfs have yet to
be confirmed in exoplanet atmospheres. An important example is chromium hydride
(CrH), which is often used to probe atmospheric temperatures and classify brown
dwarfs into spectral types. Recently, tentative evidence for CrH was reported
in the low-resolution transmission spectrum of the hot Jupiter WASP-31b. Here,
we present high spectral resolution observations of WASP-31b's transmission
spectrum from GRACES/Gemini North and UVES/VLT. We detect CrH at 5.6$\sigma$
confidence, representing the first metal hydride detection in an exoplanet
atmosphere at high spectral resolution. Our findings constitute a critical step
in understanding the role of metal hydrides in exoplanet atmospheres.
