Visible-light photocatalysis has drawn tremendous attention in the field of chemical industry owing to its considerable significance in pollution abatement and energy supply. Herein, graphitic carbon nitride/multiwalled carbon nanotubes (g-C3N4/MWCNTs) nanocomposites are fabricated using a melamine-cyanuric acid supramolecular assembly as precursor via a facile one-pot method. On one hand, the stable covalent connection between g-C3N4 and MWCNTs is built during the self-assembling process of the supramolecular precursor, which intensifies catalytic and adsorption abilities. On the other hand, MWCNTs confer both active sites and a radially single-track electron transfer path, which is conducive to restraining the combination of photogenerated carriers. The g-C3N4/MVVCNTs nanocomposite with the optimal MWCNTs content can completely degrade rhodamine B (RhB) solution within 1 h, and the degradation rate is improved similar to 6.6 times compared with pure g-C3N4. This facile strategy for fabricating efficient and stable visible-light-driven photocatalysts may hold great promise in large-scale water treatment.