The Second Battle of the Masurian Lakes or Winter Battle of the Masurian Lakes, known in Germany as the Winter Battle in Masuria and in Russia as the Battle of Augustowo, was the northern part of the Central Powers' offensive on the Eastern Front in the winter of 1915 during World War I. The offensive was intended to advance beyond the Vistula River and perhaps knock Russia out of the war. It was the last major battle fought on German soil during World War I.

The Eastern Front or Eastern Theater of World War I was a theater of operations that encompassed at its greatest extent the entire frontier between Russia and Romania on one side and Austria-Hungary, Bulgaria, the Ottoman Empire, and Germany on the other. It ranged from the Baltic Sea in the north to the Black Sea in the south, involved most of Eastern Europe, and stretched deep into Central Europe. The term contrasts with the Western Front, which was being fought in Belgium and France.