We demonstrate plasmon-mechanical coupling in a metalized nanomechanical oscillator. A coupled surface plasmon is excited in the 25 nm wide gap between two metalized silicon nitride beams. The strong plasmonic dispersion allows the nanomechanical beams thermal motion at a frequency of 4.4 MHz to be efficiently transduced to the optical transmission, with a measured displacement spectral density of 1.11 x 10(13) m/Hz(1/2). When exciting the second-order plasmonic mode at lambda = 780 nm we observe optical-power-induced frequency shifts of the mechanical oscillator. Our results show that novel functionality of plasmonic nanostructures can be achieved through coupling to engineered nanoscale mechanical oscillators.