We report the discovery of a 50,000-y-old Neandertal tar-hafted flint tool found off the present-day Dutch coastline. The production of birch tar adhesives was a major technological development, demonstrating complex Neandertal technology and advanced cognitive ability. The rarity of Middle Paleolithic adhesive finds makes each new discovery crucial for improving our understanding of Neandertal lifeways. We demonstrate that birch tar was a routine part of the Neandertal technological repertoire. In addition, the complex know-how required for adhesive production in northwestern Europe during Marine Isotope Stage 4 and 3 was maintained in small groups leading highly mobile lives. This suggests a degree of task specialization and supports the hypothesis that ecological risk drives the development of complex technology."