Coupling between a planar waveguide (PWG) mode in an organic single crystal and a surface plasmon polariton (SPP) mode is investigated in this paper. Anticrossing behavior appearing under the strong coupling regime was observed in the dispersion curves of the coupled modes. Although previous studies have utilized spin-coated organic waveguides, to date, there have been no reports on the use of single-crystalline waveguides. The so-called Fano line shapes have been observed in spectra in numerous studies related to PWG−SPP coupled systems, which are expected to find utility in surface plasmon resonance sensors. Meanwhile, transitions from Fano line shapes to Rabi splitting have been observed with increase in coupling and have attracted considerable attention in fundamental physics. As organic single crystals exhibit strong optical anisotropy, the influence of directional dispersions on coupling strength is expected to be significant. In the present study, we have developed angle-resolved transmission microspectroscopy that enables optical measurements of an organic single crystal with an in-plane dimension of hundreds of micrometers even in the nonradiative region. Because measuring such a small sample using conventional attenuated total reflection measurements is difficult, the transmission spectra in the nonradiative region were measured by rotating the sample in index-matching oil. Accordingly, dispersion curves reflecting the coupling between the PWG and SPP modes were obtained.