Flucytosine, CAS #2022-85-7, crystallizes in the tetragonal space group P41212 (#94) with a = 6.643768(27), c = 23.89009(10) Å, V = 1054.500(7) Å3, and Z = 8. In this work, the sample was obtained from the United States Pharmacopeial Convention (USP) Lot #R03100 and analyzed as-received. The room temperature (295 K) crystal structure was refined using synchrotron (λ = 0.412826 Å) powder diffraction data and optimized using the density functional theory (DFT). When looking down the a-axis, the crystal structure consists of multiple ribbon-like structures stacked into columns. The powder X-ray diffraction pattern of the compound has been submitted to ICDD® for inclusion in the Powder Diffraction File™ (PDF®). The agreement of the Rietveld-refined and DFT-optimized structures is good, with the largest difference in the external amine group with an overall root mean displacement of 0.056 Å. There is also evidence of unit cell expansion at higher temperatures, as the volume of the unit cell at 298 K was 1.6–1.9% greater than the two unit cells obtained at 150 K. A N–H⋯O hydrogen bond exists in the inter-ribbon region between the flucytosine molecules, resulting in a 3D hydrogen bond network.