We present a Hubble Space Telescope/Wide-Field Camera 3 near-infrared spectrum of the archetype Y-dwarf WISEPJ182831.08+265037.8. The spectrum covers the 0.9-1.7{mu}m wavelength range at a resolving power of {lambda}/{Delta}{lambda}~180 and is a significant improvement over the previously published spectrum because it covers a broader wavelength range and is uncontaminated by light from a background star. The spectrum is unique for a cool brown dwarf in that the flux peaks in the Y, J, and H bands are of near equal intensity in units of f{lambda}. We fail to detect any absorption bands of NH3 in the spectrum, in contrast to the predictions of chemical equilibrium models, but tentatively identify CH4 as the carrier of an unknown absorption feature centered at 1.015{mu}m. Using previously published ground- and spaced-based photometry, and using a Rayleigh-Jeans tail to account for flux emerging longward of 4.5{mu}m, we compute a bolometric luminosity of log(L_bol_/L{sun}^N^)=-6.50{+/-}0.02, which is significantly lower than previously published results. Finally, we compare the spectrum and photometry to two sets of atmospheric models and find that the best overall match to the observed properties of WISE1828+2650 is a ~1Gyr old binary composed of two Teff~325K, ~5MJup brown dwarfs with subsolar [C/O] ratios.