Comets are thought to be among the least modified objects in our solar system. Studying the chemistry of comets can therefore provide insight into the conditions present in the early solar system when (and where) they formed. We present an analysis of high-resolution, near-infrared spectra of long-period comet C/2020 S3 (Erasmus) obtained on UT 29 November and 1 December, 2020. The comet was ~0.5 au from the sun on both dates and was on the pre-perihelion portion of its orbit. The observations were performed at the 3m NASA Infrared Telescope Facility (IRTF) with Director’s Discretionary Time using iSHELL, a high-spectral-resolution spectrograph with sensitivity in the 1.15-5.4 μm wavelength range. We targeted multiple transitions of H2O, C2H6, H2CO, OH, CH3OH, CH4, HCN, C2H2, CO, OCS, and NH3 to characterize the volatile composition of the comet. Observations were obtained using the 15″ x 0.″75 slit, providing a resolving power (λ/Δλ) ~45,000. We report rotational temperatures (Trot), production rates (Q), and mixing ratios (with respect to H2O and C2H6), as well as spatial profiles of emission for volatiles and dust (continuum). We discuss possible asymmetries in the outgassing behavior and place the composition of C/2020 S3 (Erasmus) into context through comparison with previous compositional measurements of comets at near-infrared wavelengths. This work is supported by the NASA Postdoctoral Program administered by USRA. Various co-authors gratefully acknowledge support by NSF AARG and by NASA Earth and Space Science Fellowship, EW, SSW, and SSO Programs.