A novel molecular heterospin system composed of high-spin Mn-II (S = 5/2) and (3,5-dichloro-4-pyridyl)bis(2,4,6-trichloro-phenyl)methyl radical (PyBTM; S = 1/2) was developed. The Mn-II ion formed distorted octahedral coordination geometry with two PyBTM radicals coordinated in the trans configuration. Magnetic studies disclosed an intramolecular antiferromagnetic exchange interaction between the spins on PyBTM and Mn-II with J(R-Mn)/k(B) of -9.7 K. The antiferromagnetic interaction was confirmed by broken-symmetry density functional theory calculations. This study reveals the definite intramolecular exchange interaction in the Mn-II-triarylmethyl radical system for the first time.