Flensburg Firth or Flensborg Fjord (; ), is the westernmost inlet of the Baltic Sea. It forms part of the border between Germany to the south and Denmark to the north. Its length is either 40 or 50 km, depending to the definition of its limits. It has the largest surface of all Förden and East Jutland Fjorde, which are a special type of inlets, different from geological fjords.