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U-Si intermetallic compounds are of considerable interest for their applications as accident-tolerant nuclear fuels. Here we present low-temperature heat capacity (LTHC) measurements of one of the U-Si phases, U3Si5, using a Quantum Design Physical Properties Measurement System (PPMS) from 2.4 to 397.4 K. We observed an upturn in Cp/T (T) below 10 K and have attributed this behavior to potential spin-fluctuations (SF) with an SF temperature (Tsf) of 27 K. An enhancement of LTHC was also observed, as manifested by a large electronic heat capacity coefficient (γel) of 342.9 mJ/mol•K2. From the heat capacity data, the following thermodynamic parameters were determined: the characteristic Debye temperature ( θ D ) over the temperature range 30 – 397 K is 177 ± 2 K, and the standard entropy ( Δ 0 298 . 15 S ∘ ) is 283.3 ± 5.7 J•mol−1•K−1 (equivalent to 35.4 ± 0.7 J•mol−1•atom−1•K−1). Combined with our previously measured formation enthalpy ( Δ f H e l ∘ ) of U3Si5, the Gibbs free energy of formation of U3Si5 from the elements ( Δ f G e l ∘ ) was determined to be –45.2 ± 9.0 kJ•mol−1•atom−1.