Efficient nonlinear conversion requires that interacting optical waves maintain a consistent phase relationship when traveling in a medium despite its dispersion. Birefringent phase-matching, which is often used to compensate for the dispersion, is not applicable to optically isotropic nonlinear materials. Here, we present a one-dimensional photonic crystal structure that allows the propagation of optical surface waves, both at the fundamental and third-harmonic frequencies, as an efficient medium for phase-matched third-harmonic generation. A unique advantage of this structure is that the effective refractive indices for the surface waves are similar to the refractive index of air at both frequencies. This allows phase-matching between the first and third harmonics, and a visible collinear beam of the third harmonic is produced at the prism-coupled output. Moreover, these optical surface waves propagate over long distances even if a lossy nonlinear nanofilm is deposited onto the photonic crystal surface. We provide experimental results for third-harmonic generation at a wavelength of 410 nm for a bare dielectric Ta2O5/SiO2 multilayer structure and for the same structure coated with a 15-nm GaAs film.