The Łódź insurrection (Polish: Powstanie łódzkie), also known as the June Days (Polish: Powstanie czerwcowe), was an uprising by Polish workers in Łódź against the Russian Empire between 21 and 25 June 1905. This event was one of the largest disturbances in the Russian-controlled Congress Poland during the Russian Revolution of 1905. Poland was a major center of revolutionary fighting in the Russian Empire in 1905–1907, and the Łódź insurrection was a key incident in those events.
For months, workers in Łódź had been in a state of unrest, with several major strikes having taken place, which were forcibly suppressed by the Russian police and military. The insurrection began spontaneously, without backing from any organized group.