The 'Battle of Modlin' was a battle between German and Polish forces for the city of Modlin during the 'Weiss' (i) invasion of Poland (13/29 September 1939).

The fortress of Modlin, on the confluence of the Vistula and Narew rivers some 30 miles (50 km) to the north of Warsaw, was initially the headquarters of Generał brygady Emil Krukowicz-Przedrzymirski’s Armia 'Modlin' until this formation was compelled to fall back to the east under severe German pressure. Between 13 and 29 September, it served as a defensive citadel for the Polish forces commanded by Generał brygady Wiktor Thommée. The fighting was linked closely with the strategic situation of the 'Battle of Warsaw'.

The Polish forces defending the fortress included the Śmierć armoured train and the Modlin anti-aircraft battery, which was credited with the destruction of more German warplanes than any other Polish unit in the entire September campaign.

The fortress and the little town of Modlin were the centre of a Polish base held by four Polish infantry divisions and a number of smaller units, and comprised areas on each side of the Vistula and Narew rivers. This area was assaulted from the north by General Adolf Strauss’s II Corps of Generaloberst Günther von Kluge’s 4^th Army using Generalmajor Werner Kempf’s Panzerdivision 'Kempf' in the north-west and Generalleutnant Eccard Freiherr von Gablenz’s 32^nd Division in the north-east, and from the south by General Wilhelm Ulex’s X Corps of General Johannes Blaskowitz’s 8^th Army using Generalleutnant Johann Pflugbeil’s 221^st Division and Generalleutnant Kurt von Briesen’s 30^th Division in the south-west, reinforced by General Hermann Hoth’s XV Corps (mot.) of General Walter von Reichenau’s 10^th Army arriving from the south with formations including Generalleutnant Georg Stumme’s 2^nd leichte Division and Generalleutnant Joachim Lemelsen’s 29^th Division (mot.).

The fortress of Modlin capitulated on 29 September, and was thus one of the last such Polish defensive installations to lay down its arms, and in the process surrendered 24,000 men.