A simple, facile in situ reduction approach is reported for the synthesis of Pd-nanoparticle-decorated phosphotungstic acid (PTA)-MIL-100(Fe) nanocomposites (Pd-H₃PW₁₂O₄₀-MIL-100(Fe), denoted Pd-PTA-MIL-100(Fe)). During the in situ synthesis, PTA is encapsulated into the matrix of MIL-100(Fe) and serves as a UV-switchable reducing agent, resulting in highly dispersed Pd NPs. Using the photocatalytic degradation of pharmaceuticals and personal care products as model reactions, the ternary Pd-PTA-MIL-100(Fe) hybrids exhibited enhanced photocatalytic activity compared with their foundation matrices, the binary PTA-MIL-100(Fe) nanocomposite. Based on photoelectrochemical analyses, the improved photocatalytic performance can be attributed to the well-known electronic conductivity of the Pd NPs, the fast electron transport of PTA, the intense visible-light absorption of MIL-100(Fe), and the matched energy levels of the three components: MIL-100(Fe), PTA, and Pd NPs. Importantly, almost no Fe and W ions were leached from the samples during the reaction, demonstrating the photostability of the Pd-PTA-MIL-100(Fe) composite. In addition, possible photocatalytic reactions mechanisms have also been investigated.