[en] Graphical abstract: The efficient charge transfer occurred at the interface of BiIO_4/Bi_2MoO_6 heterojunction results in the efficient separation of photoexcited electron–hole pairs and promotes the photocatalytic activity. - Highlights: • BiIO_4/Bi_2MoO_6 composites were synthesized by a one-step hydrothermal method. • The BiIO_4/Bi_2MoO_6 composite exhibits much better photoelectrochemical performance. • The highly improved photocatalytic activity is attributed to heterojunction structure. • Holes (h"+) are the main active species in the photodegradation process of RhB. - Abstract: A novel BiIO_4/Bi_2MoO_6 heterojunction photocatalyst has been successfully developed by a one-step hydrothermal method for the first time. It was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and diffuse reflection spectroscopy (DRS). Compared to pure BiIO_4 and Bi_2MoO_6, the BiIO_4/Bi_2MoO_6 composite exhibits the much better photoelectrochemical performance for Rhodamine B (RhB) degradation and photocurrent (PC) generation under visible light irradiation (λ > 420 nm). This enhancement on visible-light-responsive photocatalytic activity should be attributed to the fabrication of a BiIO_4/Bi_2MoO_6 heterojunction, thus resulting in the high separation and transfer efficiency of photogenerated charge carriers. The supposed photocatalytic mechanism dominated by holes (h"+) was verified by the photoluminescence (PL) spectroscopy, electrochemical impedance spectra (EIS) and active species trapping experiments