In this work, graphitic carbon nitride (g-C3N4) was doped with boron by thermal treatment with NaBH4. We have shown that the morphology of the modified polymer gradually changes from graphitic-like to amorphous, with increasing amounts of NaBH4 used for annealing. On the other hand, the boron doping has a very small influence on the band gap energy, as well as on the positions of the valence and conduction band edges of the polymer, but strongly improves the activity of g-C3N4 in the photocatalytic reduction of oxygen and degradation of methyl orange (8-fold increase of the degradation rate constant with respect to that of pure g-C3N4). The enhancement of the photocatalytic properties was explained by the increased rate of charge transport, both in the polymer plane and across the film, due to formation of N–B–N bonds between the adjacent heptazine units and the interlayer Coulomb interactions. The best photocatalytic activity was obtained for samples prepared from a mixture of g-C3N4 and NaBH4 at a moderate weight ratio (4:1), while increased amounts of NaBH4 led to significant and unfavorable changes in both the morphology and molecular structure of the polymer.