Synchrotron X-ray diffraction (XRD) and neutron powder diffraction (NPD) were used to determine the structure of Cu(NCS)₂(pyz) (pyz = pyrazine = C₄N₂H₄), which consists of a stacking of Cu-pyz-Cu chains. While NPD measurements showed no evidence of long-range magnetic ordering, the temperature dependence of the magnetic susceptibility and magnetization suggests that the system can be adequately described on the local scale as a spin-1/2 antiferromagnet (AFM) chain with an intrachain exchange interaction J/k_B = -8 K (∼0.7 meV). Comparison of isothermal magnetization data acquired up to 30 T at 1.6 K to a linear chain model shows excellent agreement, making this material a nearly ideal example of an isotropic Heisenberg AFM chain.