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WASP-121b is a transiting gas giant exoplanet orbiting close to its Roche limit, with an inflated radius nearly double that of Jupiter and a dayside temperature comparable to a late M dwarf photosphere. Secondary eclipse observations covering the 1.1–1.6μm wavelength range have revealed an atmospheric thermal inversion on the dayside hemisphere, likely caused by high-altitude absorption at optical wavelengths. Here we present secondary eclipse observations made with the Hubble Space Telescope Wide Field Camera 3 spectrograph that extend the wavelength coverage from 1.1μm down to 0.8μm⁠. To determine the atmospheric properties from the measured eclipse spectrum, we performed a retrieval analysis assuming chemical equilibrium, with the effects of thermal dissociation and ionization included. Our best-fitting model provides a good fit to the data with reduced χ2ν=1.04⁠. The data diverge from a blackbody spectrum and instead exhibit emission due to H− shortward of 1.1μm⁠. The best-fitting model does not reproduce a previously reported bump in the spectrum at 1.25μm⁠, possibly indicating this feature is a statistical fluctuation in the data rather than a VO emission band as had been tentatively suggested. We estimate an atmospheric metallicity of [M/H]=1.09+0.57−0.69⁠, and fit for the carbon and oxygen abundances separately, obtaining [C/H]=−0.29+0.61−0.48 and [O/H]=0.18+0.64−0.60⁠. The corresponding carbon-to-oxygen ratio is C/O=0.49+0.65−0.37⁠, which encompasses the solar value of 0.54, but has a large uncertainty.