In this study we present new seawater data of 236U and 238U sampled in the North Sea in 2010. The North Sea has been and is still receiving a considerable input of anthropogenic radionuclides from nuclear reprocessing facilities located in La Hague (France) and Sellafield (Great Britain). It therefore represents an important source region for oceanographic tracer studies using the transient signal of anthropogenic 236U. A proper knowledge of the sources of 236U is an essential prerequisite for such tracer studies. The 236U data set presented in this study covers the transition regions of the North Sea to the Atlantic Ocean, to the Baltic Sea, and upstream the Elbe River. It is discussed in the context of available 236U data from the literature. Our results show that both 236U concentrations and 236U/238U ratios in surface waters of the North Sea can be explained by simple binary mixing models implying that 236U behaves conservatively in seawater. We further show that the input of 236U by the Elbe River is negligible, while there might be a maximum input of 12 g/yr via the Baltic Sea. The results of the mixing models suggest that this still unidentified 236U contamination could be supplied by fresh water input.