This Letter reports the first measurement of the 235 U νe energy spectrum by PROSPECT, the Precision Reactor Oscillation and Spectrum experiment, operating 7.9 m from the 85 MW th highly-enriched uranium (HEU) High Flux Isotope Reactor. With a surface-based, segmented detector, PROSPECT has observed 31678 ± 304 (stat.) νe-induced inverse beta decays (IBD), the largest sample from HEU fission to date, 99 % of which are attributed to 235 U. Despite broad agreement, comparison of the Huber 235 U model to the measured spectrum produces a χ 2 /ndf = 51.4/31, driven primarily by deviations in two localized energy regions. The measured 235 U spectrum shape is consistent with a deviation relative to prediction equal in size to that observed at low-enriched uranium power reactors in the νe energy region of 5-7 MeV.Reactor ν e experiments have been central to the understanding of neutrinos, including the first observation of ν e [1], the discovery of ν e oscillations [2], observation of ν e produced within the Earth [3], and the measurement of the neutrino mixing angle θ 13 [4][5][6]. Most of these experiments were located at low-enriched uranium (LEU) nuclear power reactors where more than 99 % of emitted ν e come from the beta decay of fission products of four isotopes ( 235 U, 238 U, 239 Pu, and 241 Pu). At power reactors, the emitted ν e flux and spectrum evolve over time as the isotopic composition changes in the fuel cycle. Comparisons between theoretical predictions and experimental results reveal a ∼6 % global flux deficit [7-10] and disagreement of the energy spectrum [11][12][13][14] and flux-evolution [15,16]. Explanations for these possibly independent phenomena may lie in the complex nuclear physics of reactors [17][18][19][20][21][22][23][24], physics beyond the Standard Model such as eV-scale sterile neutrinos [8], or both [25-27]. New experiments at compact-core, highly enriched uranium (HEU) research reactors enable short baseline searches for sterile neutrino oscillations and the measurement of the nearly time-independent emission of ν e from 235 U fission [28][29][30]. PROSPECT has recently reported a search for sterile neutrinos at the High Flux Isotope Reactor (HFIR) [31]. This Letter reports the first measurement of the ν e energy spectrum from HFIR by the PROSPECT experiment and the higheststatistics 235 U spectral measurement since the ILL experiment observed ∼5000 ν e candidates in 1981 [32].Located at Oak Ridge National Laboratory, HFIR is an 85 megawatt thermal (MW th ) HEU research reactor. The cylindrical reactor core (diameter: 0.435 m, height: 0.508 m) contains 93 % 235 U enriched fuel, leading to a ∼99 % 235 U fission fraction. Each 24-day operating cycle uses fresh fuel, minimizing 239 Pu and 241 Pu production. The PROSPECT detector is deployed in a ground-level room at a center-to-center distance of (7.9 ± 0.1) m from the reactor core. The core center is located 40°below the horizontal and the surrounding building provides less than one meter-water-equivalent of concrete overburden.