We report near-infrared spectroscopic data for the Seyfert 2 galaxy Mrk 78, taken with the LIRIS near-infrared camera/spectrometer at the William Herschel Telescope (WHT). The long-slit spectra clearly show extended emission. The resolution and depth of the near-infrared spectra allow the examination of its morphology and ionization regions, and a direct comparison with similarly deep visible spectra. The emission-line ratios obtained are used to derive the extinction toward the nucleus. The detection of strong features such as [Fe II], H2, hydrogen recombination lines, and the coronal [Si VI] λ1.962 line is used to study the kinematics and excitation mechanisms occurring in Mrk 78, revealing that despite the strong radio-jet interaction present in this object, photoionization from the active nucleus dominates the narrow-line region emission, while UV fluorescence is the source of the H2 emission. Lines with extended emission yield velocity distributions with an amplitude of about 600 km s-1, the consequence of an eastern lobe moving away from us plus a western lobe with the opposite contribution. We used the photoionization code CLOUDY to recreate a typical narrow-line region, to derive the ionization parameter, and to compare our spectral data with diagnostic diagrams.