Nanoporous gold films are prepared using a dealloying method and form a sponge type bicontinuous network. As the structure sizes are below 50 nm, the material forms an effective medium with a negative dielectric constant for near infrared light. The dispersion relation of the propagating surface plasmons on the air/nanoporous gold interface is determined from reflection measurements in the Kretschmann configuration. A characteristic red-shift by ca. 0.85 eV compared to surface plasmons on solid gold layers is observed. The results are compared with calculated dispersion relations applying the Bruggeman effective medium theory for the nanoporous gold films.