Dyes are important in many industrial sectors but represent a threat if they are discharged in waste water. In this global concern, novel TiO2-polymer nanocomposites were designed for the removal of methyl orange (MO), a model pollutant. In this process, polyaniline (PANI) was prepared by in situ polymerization of aniline on TiO2 nanoparticles grafted with 4-diphenylamine (DPA) groups from diazonium salt. PANI loading was very high on diazonium-modified TiO2 and withstood leaching in polar solvents compared to the low loading and poor adhesion noted for bare TiO2. TiO2-DPA-PANI efficiently catalyzed the degradation of MO in aqueous media under UV light. Total discoloration of MO solutions was achieved without any spectral evidence for organic byproducts thus suggesting complete mineralization of the dye. The degradation rate constant (K-app) was 0.133 min(-1) for TiO2-DPA-PANI nanocomposite, much higher than 0.059 and 0.085 min(-1) found for TiO2 and TiO2-PANI, respectively. This clearly stresses the role of PANI for improving the activity of the photocatalyst and for protecting TiO2 under UV light. Indeed, TiO2-DPA-PANI could be recycled 5 times without any loss in the photocatalytic activity, whereas TiO2-PANI could be recycled only 3 times and the bare TiO2 reused once. This work highlights the major role of diazonium salts in the design of novel photocatalysts for the mineralization of organic dye pollutants.