Simultaneous photocatalytic Escherichia coli (E. coli) inactivation and methylene blue (MB) degradation in water are achieved by a composite containing MoO$_3$ and TiO$_2$ under UV and UV–visible irradiations. The MoO$_3$/TiO$_2$ composites are synthesized via an incipient wet impregnation method with varying concentrations of MoO$_3$ from 0 to 10 wt% in TiO$_2$. Under both UV and UV–visible irradiations, the photocatalytic results indicate that the MoO$_3$/TiO$_2$ composites destruct the bacteria significantly at higher rates than the unmodified TiO$_2$ and pure MoO$_3$. Particularly, 5 wt% MoO$_3$/TiO$_2$ sample exhibits improved photocatalytic activity for the simultaneous E. coli destruction and MB degradation under UV–visible irradiation, replacing TiO$_2$ which is photoactive only under UV irradiation. The Brønsted acidity and OH radical concentration increase with MoO$_3$ concentration on TiO$_2$ along with a heterojunction effect are the responsible factors for yielding high photocatalytic activity of MoO$_3$/TiO$_2$ composite under UV–visible irradiation. Particularly, 99% of degradation of 20 ppm MB is achieved with a very low catalyst dose of 0.6 g l$^{-1}$ at a very less time of 20 min using 5% MoO$_3$/TiO$_2$ sample under UV–visible irradiation. Consequently, MoO$_3$/TiO$_2$ composite is a potential candidate for E. coli inactivation and MB degradation in water, signifying its prospective for the purification of water.