3-(Phenyl)-1-(pyrid-2-yl)-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl (2) demonstrates the first example of polymorphism in the family of Blatter radicals. Two polymorphs, 2α and 2β, have been identified and characterized by single crystal X-ray diffractometry and magnetic susceptibility measurements to investigate their magnetism–structure correlations. Both polymorphs form one-dimensional (1D) π stacks of evenly spaced radicals with distinctly different π–π overlap modes. Within the 1D π stacks, radicals are located at evenly interplanar distances, 3.461 Å for 2α and 3.430 Å for 2β. Magnetic susceptibility studies indicate that both polymorphs exhibit antiferromagnetic interactions inside their 1D π stacks. The magnetic susceptibility data are best interpreted in terms of a regular chain model of antiferromagnetically coupled quantum spins (H = -Σi Si.Si+1) with exchange-interactions of J/kB = −36.7(3) K (−25.5(2) cm−1) for 2α and J/kB = −72(3) K (−50(2) cm−1) for 2β. For polymorph 2β, a crossover on the magnetic susceptibility around 20 K suggests the presence of a phase transition, which might be related to dimerization of the radicals along the chain. DFT calculations support the experimental structure–magnetism results and the antiferromagnetic nature of the local interactions between radicals within the 1D π stacks.