An organic single-crystal optical resonator that maintains its optical performance even upon mechanical bending remains a challenging target. Here, the first flexible organic crystal (FOC) that works as a tunable optical resonator as a function of bending angle is developed. A fluorescent pi-conjugated molecule, cyano-substituted oligo(phenylenevinylene)s, is allowed to crystallize slowly, yielding a discrete rod-shape microcrystal (FOCCOPV). FOCCOPV not only works as an efficient optical waveguide along its longitudinal direction with the optical loss coefficient of 0.249 dB mu m(-1) but also works as a Fabry-Perot type optical resonator along its lateral direction. The resonance wavelength is finely tunable toward the shorter wavelength region by the mechanical bending of FOCCOPV plausibly due to the Poisson effect, namely, the contraction of the lateral dimension of the crystal by the longitudinal strain, along with the change in the refractive index. The mechanical bending and associated spectral shift is repeated for many cycles without deterioration in their mechanical and optical properties.