The 'Battle of Kępa Oksywska' was fought between German and Polish forces on the Oksywie Heights outside the city of Gdynia within the context of the 'Weiss' (i) German invasion of Poland (10/19 September 1939).

The battle was one of the bloodiest of the campaign, with Polish losses in men killed amounting reaching approximately 14% of all the force engaged.

On 10 September, after a series of skirmishes in the vicinity of Reda in the western outskirts of Gdynia, the Polish commander of the Land Coastal Defence forces, Generał brygady Stanisław Dąbek, found himself in a difficult position. His force, centred on the port of Gdynia, was compelled to wage a two-front war against German forces advancing both from the west and from the territory of the Free City of Danzig to the south-east. The main part of his force was facing the Germans under General Leonhard Kaupisch advancing from the east along the shores of Gdańsk Bay. At the same time a smaller detachment of the Polish forces prepared to make a stand in the area of Oksywie, where the German forces managed to isolate the Poles in the nearby Hel peninsula. Thus Gdynia was besieged.

Although the Polish forces managed to halt the German war of movement, and the fighting came to resemble the static warfare of World War I on the Western Front, the besieged garrison was suffering heavy losses and had an acute need to shorten its front. As there was no chance of relief for the besieged Polish force by forces from the Polish mainland, the heights offered a good defensive position. Although completely unprepared for defence, the area was densely forested and made German penetration and aerial and naval bombardments much more difficult.

In order to spare the city from destruction in the otherwise likely event of urban warfare, on 12 September Dąbek ordered his entire force to withdraw to Oksywie Heights to the north-east of the town, thus abandoning Gdynia. Until 14 September the Polish force, by then reduced to about 9,000 men, gradually withdrew to the area, along with as many as 140 heavy machine guns, 14 mortars, 23 pieces of artillery and a large number of civilian refugees. Massed in an area no greater of no more than 1.54 sq miles (4 km²), the Poles were able to inflict heavy losses on the numerically and technically superior Germans. Until 19 September there were no less than 110 actions and skirmishes.

However, constant aerial bombardment and the growing shortage of both supplies and reinforcements finally forced Dąbek to order a ceasefire on 19 September. By then the Polish casualties amounted to some 2,000 dead and 7,000 wounded, and there were hardly any Poles left unwounded. Dąbek then committed suicide.