We study a surface plasmon polariton mode that is strongly confined in the transverse direction and propagates along a periodically nanostructured metal-dielectric interface. We show that the wavelength of this mode is determined by the period of the structure, and may therefore, be orders of magnitude smaller than the wavelength of a plasmon-polariton propagating along a flat surface. This plasmon polariton exists in the frequency region in which the sum of the real parts of the permittivities of the metal and dielectric is positive, a frequency region in which surface plasmon polaritons do not exist on a flat surface. The propagation length of the new mode can reach a several dozen wavelengths. This mode can be observed in materials that are uncommon in plasmonics, such as aluminum or sodium.
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