Using radial-velocity measurements from four instruments, we report the mass and density of a 2.043 ±0.069 R⊕ sub-Neptune orbiting the quiet K-dwarf Wolf 503 (HIP 67285). In addition, we present improved orbital and transit parameters by analyzing previously unused short-cadence K2 campaign 17 photometry and conduct a joint radial-velocity-transit fit to constrain the eccentricity at 0.41 ± 0.05. The addition of a transit observation by Spitzer also allows us to refine the orbital ephemeris in anticipation of further follow-up. Our mass determination, 6.26 ${}_{-0.70}^{+0.69}$ M⊕ , in combination with the updated radius measurements, gives Wolf 503 b a bulk density of $\rho ={2.92}_{-0.44}^{+0.50}$ g cm-3. Using interior composition models, we find this density is consistent with an Earth-like core with either a substantial H2O mass fraction (45 ${}_{-16}^{+19}$ %) or a modest H/He envelope (0.5% ± 0.3%). The low H/He mass fraction, along with the old age of Wolf 503 (11 ± 2 Gyr), makes this sub-Neptune an opportune subject for testing theories of XUV-driven mass loss while the brightness of its host (J = 8.3 mag) makes it an attractive target for transmission spectroscopy.