The photocatalytic activity of semiconductors can be tuned by changing their morphological or structural properties. However, a simpler and direct method is the introduction of a cocatalyst, for example V2O5 or V2O5/V4O9. In the present work, this was the cocatalyst added to SrTiO3. The deposition method was directed in such a way that the cocatalyst did not cover the surface of the SrTiO3 completely. This way, the photocatalytic process (phenol conversion) takes place at the surface of the main catalyst, while the lifetime of the generated charge carriers is increased through electron trapping via the presence of vanadium oxides. The V2O5/V4O9 cocatalyst influences the recombination processes of excited electrons in SrTiO3 by modifying the near-surface defects of SrTiO3, and it can efficiently capture electrons due to the formed heterojunction. The V4O9 content enables efficient electron transfer, as its structure can accommodate V⁴⁺ in addition to V⁵⁺. Therefore, a mixed-phase semiconductor is more suitable as a cocatalyst than a single-phase semiconductor. In this work, the photocatalytic activity of SrTiO3 was investigated in the presence of V2O5 (0–20 wt.%). It was found that all the samples that contained the cocatalyst showed higher photocatalytic activity than the unmodified SrTiO3. The sample containing 10 wt.% of cocatalyst performed ~5.4 times better than pristine SrTiO3 (35.87 µmolphenol/gcatalyst, vs. 7.74 µmolphenol/gcatalyst). This sample also contains a relatively high amount of V4O9 compared to the other samples, in addition to V2O5, which may be the main reason for the enhanced photocatalytic performance.