We present and confirm TOI-1751 b, a transiting sub-Neptune orbiting a slightly evolved, solar-type, metal-poor star (Teff = 5996 ± 110 K, log(g)=4.2 + 0.1, V = 9.3 mag, [Fe/H] = −0.40 ± 0.06 dex) every 37.47 days. We use TESS photometry to measure a planet radius of 2.77-0.07+0.15 R. We also use both Keck/HIRES and APF/Levy radial velocities (RV) to derive a planet mass of 14.5-3.14+3.15M, and thus a planet density of 3.6 ± 0.9 g cm−3. There is also a long-period (∼400 days) signal that is observed in only the Keck/HIRES data. We conclude that this long-period signal is not planetary in nature and is likely due to the window function of the Keck/HIRES observations. This highlights the role of complementary observations from multiple observatories to identify and exclude aliases in RV data. Finally, we investigate the potential compositions of this planet, including rocky and water-rich solutions, as well as theoretical irradiated ocean models. TOI-1751 b is a warm sub-Neptune with an equilibrium temperature of ∼820 K. As TOI-1751 is a metal-poor star, TOI-1751 b may have formed in a water-enriched formation environment. We thus favor a volatile-rich interior composition for this planet.