The 'Battle of Grudziądz' was fought between German and Polish forces in the first days of the German 'Weiss' (i) invasion of Poland (1/4 September 1939).

The Polish city of Grudziądz (Graudenz in German) lies on the Vistula river and in 1939 was on the border of the 'Polish corridor'. The city accommodated the headquarters of the Polish 16^th Division, as well as the Centre of Cavalry Training. Moreover, it played a crucial role as a strongpoint in the defence of the Vistula river line and the route securing the line of retreat for the Polish divisions of the 'Pomorze' Armia under the command of Generał dywizji Władysław Bortnowski, engaged on the western bank of the river. Among units stationed in the 'Polish corridor' were the 9^th, 15^th and 27^th Divisions, together with the Pomeranian Cavalry Brigade from Bydgoszcz. As the Polish high command had planned an armed intervention in the Free City of Danzig, the 27^th Division and the Pomeranian Cavalry Brigade had been relocated farther to the north in the middle of August 1939, to the area of Chojnice and Starogard Gdański. On 1 September, as 'Weiss' (i) began, both of these remained in the 'Polish corridor' and thus very vulnerable to a German attack. To the east of the Vistula river, along the line stretching from Grudziądz to Lidzbark (Lautenburg in German) stood General Günther von Kluge’s German 4^th Army of Generaloberst Fedor von Bock’s Heeresgruppe 'Nord'. Grudziądz itself was defended by the Operational Group East under the command of General brygady Mikołaj Bołtuć and comprising Pułkownik Tadeusz Lubicz-Niezabitowski’s 4^th Division from Toruń (Thorn in German) and Pułkownik Stanisław Świtalski’s local 16^th Division. Both formations were part of the 'Pomorze' Armia.

The Grudziądz area was attacked on 1 September by General Nikolaus von Falenhorst’s XXI Corps as Generalleutnant Kuno-Hans von Both 's 21^st Division and Generalleutnant Woldemar Freiherr Grote’s 218^th Division drove back the Polish lines behind the small Osa river to the east of Grudziądz. The main German attack was concentrated on the Polish left wing, which was defended by the 16^th Division. In the area of Łasin, German units were halted, but another attack, near Dąbrówka Królewska, was successful. After crossing the Osa river, the Germans seized a bridgehead near Bielawki. During the afternoon of the same day, after receiving reinforcements the Germans continued their attack, which was finally halted by the Poles at about 19.00. During the night of 1/2 September, Bołtuć led the 4^th Division in a counterattack against the 218^th Division in order to push the invaders back behind the Osa river. The Polish forces gained some ground, but their the attack was finally repulsed.

At 08.00 on 2 September, the Germans began an assault to drive the 16^th Division farther back. After an artillery bombardment, the German infantry moved into action and expanded their bridgehead. The Polish situation was particularly difficult on the western wing of their defence line and, as a result, the 66^th and the 64^th Regiments of the 16^th Division had to retreat toward the railway linking Grudziądz and Jabłonowo Pomorskie. To save the situation, Bortnowski ordered the Polish air force’s 142^nd Squadron to bomb the German positions. The bombing resulted in no significant gains for the Poles, and in the afternoon of that day, the Poles began to retreat. Panic broke out in some Polish units as the German forces, supported by local 'fifth-column' elements, attacked the 208^th Reserve Regiment.

Even so, Bołtuć ordered Lubicz-Niezabitowski’s 4^th Division to counterattack in order to help the 16^th Division that was under heavy German pressure. The 4^th Division attacked from Radzyń Chełmiński toward Mełno at 20.00. Initially, the Germans fell back toward Annowo and Gruta, which was recaptured by the Poles at midnight. Furthermore, the Polish 65^th Regiment recaptured Nicwałd, but the Poles were halted before they reached their original defensive positions along the Osa river. During that night, Bołtuć dismissed Świtalski, who was unable to control his men, and replaced him with Pułkownik Zygmunt Bohusz-Szyszko.

The next day, 3 September, began with a massive German assault on the Polish positions. At the same time Bołtuć received news that the Polish divisions on the western bank of the Vistula river were facing defeat, and also that the German forces had already crossed the Vistula river farther to the south. This forced Bołtuć to pull back his forces in order to avoid encirclement. The Polish defenders destroyed the bridges over the Vistula river and retreated to the south-east in the direction of the Drwęca river, where they adopted new defensive positions. Grudziądz itself was abandoned during the early afternoon of 3 September. After a few hours, elements of Oberst Heinrich von Behr’s 45^th Infanterieregiment entered the city, but the German main forces did not complete their seizure of Grudziądz until the morning of 4 September.