Visible-light responsible Mn-N-codoped TiO2 nanocrystal photocatalysts were synthesized for the first time by a simple hydrothermal synthesis method. X-ray powder diffraction (XRD) measurement indicated that all of the photocatalysts have an anatase crystallite structure and the increase of the doping concentration had less effect on the structure and particle size. Comparing to N-doped TiO2, a shift of the absorption edge of Mn-N-codoped TiO2 to a lower energy and a stronger absorption in the visible light region were observed. The Mn-N-codoped TiO2 showed a higher photocatalytic reactivity than undoped TiO2 and N-doped TiO2 for the photodegradation of rhodamine B (RhB) under visible light irradiation. The doping concentration had an optimal value, according to the highest photocatalytic activity. This suggested that Mn and N codoping has important effects on the improvement of visible-light responsible photocatalytic activity.