The Battle of ProstkI was fought near Prostki (German: Prostken), Duchy of Prussia (today in Ełk County, Poland) on October 8, 1656, between forces of the Polish–Lithuanian Commonwealth and allied Crimean Tatars commanded by hetman Wincenty Gosiewski on one side, and on the other allied Swedish and Brandenburg forces commanded by Prince Georg Friedrich of Waldeck, reinforced by the cavalry of Prince Bogusław Radziwiłł.

Polish-Lithuanian commanders then decided to invade the Duchy of Prussia, which had been a vassal state to the Kingdom of Poland and which, as Brandenburg-Prussia, had been an ally of the Swedish Empire (see Treaty of Königsberg (1656).

Waldeck reached Prostken (Prostki) on October 6 and camped on the eastern bank of the Ełk River, near a bridge that he planned to defend.

Gosiewski, who enjoyed numerical superiority over the enemy, decided to make the Prussians abandon their positions behind the river and destroy them in an open field.

Prince Waldeck, with 500 soldiers, managed to escape, as the Tatars and the Lithuanians were too busy looting the Prussian camp.

The Great Elector rejected this, which resulted in widespread looting and destruction of the southeastern corner of the Duchy of Prussia, together with the neighboring Polish counties of Wizna and Rajgród.