The presence of anthropogenic nanoparticles (NPs) in the aquatic environment has become an emerging concern in terms of environmental and health safety. In the present study, we assessed the presence of Ag-bearing, Ti-bearing, and Ce-bearing NPs in the Barcelona catchment area, including the Besòs River basin and the Barcelona coast, and in the Ebro River Delta, using single particle inductively coupled plasma mass spectrometry (sp-ICP-MS). Ti-NPs and Ce-NPs were ubiquitously detected in surface waters, and their presence was related to a high natural background. Concentrations of Ti-NPs ranged from 23.2 × 106 to 298 × 106 Ti-NPs/L, with high concentrations being detected in areas with little anthropogenic pressure, while the presence of nanosilver (17.9 × 106 to 45.1 × 106 Ag-NPs/L) in the analyzed rivers was limited to certain hotspots close to wastewater treatment plants discharge points. The concentrations of Ce-NPs in the river ranged from 18.1 × 106 to 278 × 106 NPs/L, and they were related to the natural occurrence of the mineral Monazite-(Ce). Overall, the concentrations of these nanomaterials in the Barcelonan coast were significantly attenuated by river-sea environmental dilution. Nevertheless, Ce-NPs were eventually detected in some seawater samples with low levels of lanthanum-NPs, suggesting anthropogenic inputs of nanoCeO2, probably from atmospheric deposition.