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 allows 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 towards the nucleus. The detection of strong features such as [Fe II], H$_{2}$, hydrogen recombination lines and the coronal [Si VI]$λ$1.962 line is used to study the kinematics and excitation mechanisms occurring in Mrk 78, revealing that despite of the strong radio-jet interaction present in this object, photoionization from the active nucleus dominates the narrow line region emission, while UV fluorescence is the source of the H$_{2}$ emission. Lines with extended emission yield velocity distributions with an amplitude of about 600 km/s, 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 region, to derive the ionization parameter, and to compare our spectral data with diagnostic diagrams. Comment: 18 pages, 12 figures, 4 tables. Accepted by ApJ