Abstract 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 ρ = 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 H 2 O 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.