Photocatalytic degradation of toxic organic pollutants in aquatic environments provides an efficient, cost-effective, and sustainable approach to environmental remediation. The optimization of photocatalytic detoxification processes essentially relies not only on the capability to fine-tailor the structures and compositions of the photocatalysts but also on detailed understanding of the mechanisms that dictate the photocatalytic interfacial molecular transformations. Here we have designed and constructed a hierarchically organized suprastructure comprising TiO2@Ag nanocomposite particles supported by cellulose microfiber matrices, which serves as both an efficient photocatalyst for the photodegradation of 4-chlorophenol into mineralized small molecules and a robust substrate for plasmon-enhanced molecular spectroscopy. Such dual functionalities provide unique opportunities for us to precisely monitor, in real time, the detailed photocatalytic molecular transformations occurring at the molecule-catalyst interfaces using surface-enhanced Raman scattering as an ultrasensitive, time resolving, and molecular fingerprinting spectroscopic tool.