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We present the sensitivity of the THEIA experiment to low-energy geo- and reactor antineutrinos. For this study, we consider one of the possible proposed designs, a 17.8-ktonne fiducial volume THEIA-25 detector filled with water-based liquid scintillator placed at Sanford Underground Research Facility (SURF). We demonstrate THEIA’s sensitivity to measure the geo- and reactor antineutrinos via inverse-beta decay interactions after one year of data taking with $$11.9 × 10^{32}$$ free target protons. Considering all uncertainties on input throughout the whole analysis chain, the expected number of geo- and reactor antineutrinos is $$220^{+30}_{–24}$$ (stats+syst) and $$168^{+26}_{–24}$$ (stat+sys), respectively, after one year ofmore » data taking. The corresponding expected fit precision of a sole experiment is evaluated at 8.7% and 10.1%, respectively. We also demonstrate the sensitivity towards fitting individual Th and U contributions, with best fit values of $$N_{\text{Th}} = 40^{+26}_{–22}$$ (stat+sys) and $$N_\text{U}=180^{+30}_{–24}$$ (stat+sys). Finally, from the fit results of individual Th and U contributions, we evaluate the mantle signal to be $$S_{\text{mantle}} = 9.3±[5.2,5.4]$$ NIU (stat+sys). This was obtained assuming a full-range positive correlation $$(ρ_c ϵ[0,1])$$ between Th and U, and the projected uncertainties on the crust contributions of 8.3% (Th) and 7.0% (U).« less