Caldicellulosiruptor bescii is the most thermophilic cellulolytic bacterium capable of fermenting crystalline cellulose identified to date, and it also has a superior ability to degrade plant biomass without any pretreatment. This study is the first to assess the potential of utilizing unpretreated cattle manure (UCM) as a feedstock for hydrogen (H2) production by C. bescii at a concentration range between 2.5–50 g volatile solids (VS)/L. At 50 g VS/L UCM concentrations, H2 production ceased due to inhibition of C. bescii. To alleviate the impacts of inhibition, two strategies were adopted: (i) reduction of H2 build-up in the reactor headspace via gas sparging and (ii) adaptation of C. bescii to UCM via adaptive laboratory evolution (ALE). The former increased H2 yield by 47% compared to the control reactors, where no sparging was applied. The latter increased H2 yield by 142% compared to the control reactors inoculated by the wild type C. bescii. The UCM-adapted C. bescii demonstrated a remarkable H2 yield of 161.3 ± 1.6 mL H2/g VSadded at 15 g VS/L. This yield represents a twofold increase compared to the maximum H2 yield reported in the literature amongst fermentation studies utilizing manure as feed. At 15 g VS/L, around 73% of UCM was solubilized, and the carbon balance indicated that most of the effluent carbon was in the sugar- and acid-form. The remarkable ability of C. bescii to produce H2 from UCM under non-sterile conditions presents a significant potential for sustainable biohydrogen production from renewable feedstocks.