The enhancement of the photocatalytic activity of TiO2 nanotubes (TNTs), as a result of heterojunction formation through the addition of SnO2 nanoparticles (NPs) by a simple and straightforward one-step hydrothermal method, is shown. The structure and properties of the as-synthesised composite material are characterised using X-ray diffraction, transmission electron microscopy, field emission scanning electron microscopy, UV-Vis diffuse reflectance spectroscopy, Raman spectroscopy and photoluminescence spectroscopy. The SnO2/TNT heterojunction with extremely small SnO2 NPs adhered on the TNT surface was successfully synthesised by a one-step hydrothermal synthesis. The mechanism and factors affecting the photocatalytic activity were verified and clarified. Furthermore, the sample with 2% SnO2 precursor content has the highest photocatalytic activity with a methylene blue degradation efficiency of 74.2% for 180 min under sunlight irradiation.