The Battle of the Bzura (or Kutno or Battle of Kutno — German name) was a battle in the opening campaign of World War II during the 1939 German invasion of Poland, fought between 9 and 19 September, 1939, between Polish and German forces. Finally, the Germans outflanked the Polish forces and took all of western Poland.

It was the single largest battle in the 1939 September campaign and took place to the west of Warsaw, near the Bzura River. In it, a Polish breakout attack gained initial success but eventually faltered after a concentrated German counterattack.

The Polish plan for defense against the German invasion, Plan West, called for the defense of the borders. This was dictated more by political than military concerns, as Poles feared that the Germans, after taking over territories they lost in the Treaty of Versailles, would try to end the war and keep those territories. While defending the borders was more risky, the Poles were counting on the British and French counteroffensive (which never came). Due to this, Army Pomorze under general Władysław Bortnowski found itself in the Polish Corridor, surrounded by German forces on two fronts, and Army Poznań under general Tadeusz Kutrzeba was pushed to the westernmost fringes of the Second Polish Republic, separated both from its primary defensive positions, and from other Polish Armies.

Bzura is a river in central Poland, a tributary of the Vistula river (in Wyszogród), with a length of 166 kilometres (25th longest) and the basin area of 7,788 km².

Coordinates: 52°22′28″N 20°12′05″E / 52.3744464026°N 20.2012825012°E / 52.3744464026; 20.2012825012