The metastable helium line at 1083 nm can be used to probe the extended upper atmospheres of close-in exoplanets and thus provide insight into their atmospheric mass loss, which is likely to be significant in sculpting their population. We used an ultranarrowband filter centered on this line to observe two transits of the low-density gas giant HAT-P-18b, using the 200 Hale Telescope at Palomar Observatory, and report the detection of its extended upper atmosphere. We constrain the excess absorption to be $0.46pm0.12%$ in our 0.635 nm bandpass, exceeding the transit depth from the Transiting Exoplanet Survey Satellite (TESS) by $3.9sigma$. If we fit this signal with a 1D Parker wind model, we find that it corresponds to an atmospheric mass loss rate between $8.3^{+2.8}_{-1.9} times 10^{-5}$ $M_mathrm{J}$/Gyr and $2.63^{+0.46}_{-0.64} times 10^{-3}$ $M_mathrm{J}$/Gyr for thermosphere temperatures ranging from 4000 K to 13000 K, respectively. With a J magnitude of 10.8, this is the faintest system for which such a measurement has been made to date, demonstrating the effectiveness of this approach for surveying mass loss on a diverse sample of close-in gas giant planets.
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