A carbon nanotube (CNT)/gold nanoparticle (NP) nanocomposite was synthesized by simultaneously reducing the Au ions and depositing Au NPs on the surface of a CNT. The functional groups were investigated with Fourier transform infrared spectra. From the Raman spectra, the D-band and G-band of the CNT were identified. The deposition of nanometer-sized Au NPs on the CNT sites was observed by transmission electron microscopy. The photodegradation of methylene blue (MB) in aqueous solutions was studied using various photocatalysts, including TiO2, TiO2-SiO2, CNT/TiO2, CNT/TiO2-SiO2, Au/TiO2, CNT-Au/TiO2, and CNT-Au/TiO2-SiO2 composites. CNT addition leads to a synergic effect, improving the photoactivity of the catalysts. A possible physically based mechanism was proposed involving the reduction of electron-hole recombination and fast electron-transfer possibility.