Organic pollutants have been a significant source of concern in recent years due to their facile dissemination and harmful effects. In this work, two different metal–organic frameworks (MOFs) were initially prepared by hydrothermal treatment, namely aluminum trimesate (MIL-100(Al)) and copper trimesate (HKUST-1). These materials were subsequently submitted to a post-synthetic modification step to grow titania nanoparticles on their surface. Anatase nanoparticles with sizes around 5 nm were successfully anchored on MIL-100(Al), and the concentration of TiO2 in this sample was about 68 wt.%. This is the first time that this composite (TiO2@MIL-100(Al)) is reported in the literature. It showed an improved photocatalytic activity, removing 90% of methylene blue (kapp = 1.29 h−1), 55% of sodium diclofenac (kapp = 0.21 h−1), and 62% of ibuprofen (kapp = 0.37 h−1) after four hours of illumination with UV-A light. A significant concentration (14 µM) of reactive oxygen species (ROS) was detected for this composite. HKUST-1 showed a structural collapse during its post-synthetic modification, leading to a non-porous material and providing fewer sites for the heterogeneous nucleation of titania. This behavior led to a low concentration of rutile nanoparticles on HKUST-1 (9 wt.%). However, the obtained composite (TiO2@HKUST) also showed an improved photoactivity compared to HKUST-1, increasing the photodegradation rates evaluated for methylene blue (0.05 h−1 vs. 0.29 h−1), sodium diclofenac (negligible vs. 0.03 h−1), and ibuprofen (0.01 h−1 vs. 0.02 h−1). This work brings new insights concerning the preparation of photocatalysts by growing semiconductor nanoparticles on trimesate-based MOFs.