Grevelingen or Grevelingenmeer (Lake Grevelingen) is a closed off part of the Rhine-Meuse estuary on the border of the Dutch provinces of South Holland and Zeeland. It is situated between the islands of Goeree-Overflakkee (South Holland) and Schouwen-Duiveland (Zeeland) and was closed off as part of the Delta Works, a huge engineering project designed to protect the southwestern part of the Netherlands from flooding. The Brouwersdam, a dike connecting the two islands on the west, closes off the Grevelingen from the North Sea. The , the dike on the east, blocks the inflow of Rhine and Meuse water.