The 'Battle of Gdynia' was one of the larger battles in northern Poland during Germany’s 'Weiss' (i) invasion of Poland (8/14 September 1939).

Lying to the north-west of Danzig on the northern coast of Poland, Gdynia was in 1939 a major civilian and military port on the Baltic Sea, and was an important Polish industrial centre. Gdynia’s defence was one of the major elements in the Polish defence plan, and one of Poland’s premier formations, Generał dywizji Władysław Bortnowski’s Armia Pomorze, had been ordered orders to prevent invading German forces from breaking out of the 'Polish Corridor' that linked most of Germany with East Prussia and to defend Gdynia, Hel, Kępa Oksywska and Oksywie.

The forces defending Gdynia were grouped under the Land Coastal Defence, commanded by Pułkownik Stanisław Dąbek and the Naval Coastal Defence, commanded by Komandor Stanisław Frankowski. The Polish plan anticipated that the Germans would succeed in cutting Gdynia off from the main forces of the Armia Pomorze, so the Land Coastal Defence was tasked with defence of the coast for some eight to 10 days, by which time it would have been relieved and reinforced. The command was thus prepared for independent operation and reassigned from Armia Pomorze to the Polish navy, and Dąbek received orders directly from Kontradmirał Józef Unrug, the commander-in-chief of the Polish navy. The Polish navy was to support the coastal defence forces with the destroyer Wicher (Poland’s three other destroyers having been sent from the Baltic Sea to the UK in 'Peking'), the large minelayer Gryf and several submarines and a number of smaller surface warships.

On 1 September the Land Coastal Defence possessed a strength of about 17,000 men (increased from about 5,000 two months earlier), with 40 pieces of artillery including eight anti-aircraft guns, 34 mortars and grenade launchers, and something in the order of 400 machine guns. The force’s infantry units were placed to the west and Wejherowo, to the south of Gdynia at Redłowo, at Kartuzy, at Koleczkowo, and at Gdynia and Kępa Oksywska

The German forces tasked with the capture of Gdynia were those of General Leonhard Kaupisch, commander of the Grenzschutz-Abschnitt-Kommando 1 in eastern Pomerania and constituted part of Generaloberst Fedor von Bock’s Heeresgruppe 'Nord'. Kaupisch had under his command some 29,000 men supported by more than 300 pieces of artillery, 70 mortars and grenade launchers, and 700 machine guns. The German navy in the area consisted of two old battleships, three cruisers, 10 destroyers and a number of smaller units. Approximately 120 Luftwaffe aircraft operated in support of the German ground forces.

The 'Weiss' (i) offensive isolated the Polish coast from the Polish mainland between 4 and 8 September, and the Armia Pomorze was forced to retreat to the south-east. Units of the Land Coastal Defence has been engaged by the German forces from 1 September, although it was not until the second week of the campaign that the Germans began their direct assault on Gdynia.

Operating in the area with the pre-dreadnought battleship Schleswig-Holstein and some smaller warships, the German navy shelled the Polish positions, but only to limited effect, and were prevented from closing on the coast by the Polish coastal artillery batteries.

The German land offensive was much more successful, and the German forces had established a connection between Germany and East Prussia by 4 September. After heavy fighting near Kartuzy on 5 September, the Germans' main attack toward Gdynia began on 8 September, and after intense fighting near Puck and Wejherowo on 8 September, the Poles were compelled to make a step-by-step withdrawal toward the coast.

After the Polish positions outside Gdynia had been overrun, and given the fact that the Armia Pomorze had retreated, Dąbek decided on 10 September to abandon Gdynia in order to avoid civilian casualties should fighting reach the city, and on 12 September he therefore ordered all Polish units to retreat from Gdynia toward Kępa Oksywska, an area of the coast near Puck Bay. On 11 and 12 September the fighting near Mechlinki escalated into a full-scale battle that lasted two days, after which the Polish forces had to retreat once more.

The Germans captured Gdynia on 14 September. Kępa Oksywska had nor been readied to withstand a siege or even provisioned for the approximately 9,000 remaining Polish troops, and in the battle of Kępa Oksywska the Poles suffered the loss of about 2,000 men. After a last and unsuccessful counterattack on 19 September, Dąbek, who was now wounded, decided to issue an order to surrender and committed suicide after giving the order.

The Polish navy had been tasked with the provision of support to Gdynia in the 'Worek' submarine operation and 'Rurka' minelaying operation. The five submarines assigned to 'Worek' left their war stations early in September: three were interned in neutral ports and the other two reached the UK. One submarine-laid mine sank a German minesweeper. 'Rurka' was stymied by German air attack on 1 September.