1-Phenylbiguanide in the solid state was oxidized by iodine, and a pi-cation radical was observed as a charge-transfer complex. The dark-brown powder exhibited a charge-transfer band in the near-UV region. The electron spin resonance (ESR) spectra of the complex exhibited Lorentzian peaks with g = 2.007-2.008, which suggested an imino-centered, monocation radical. Regardless of the imino fragment with a sigma lone-pair, the cationic species was characterized as a pi-radical conjugated with the phenyl group. This is the first observation of pi-cation radical complexes of guanidine derivatives in the solid state.