On 22 June 1941, German troops attacked the Soviet Union in Operation Barbarossa. They soon reached Ukraine and had reached Kiev by late August. In September, Rudniev formed a partisan group in Putyvl. In October, Rudniev and Sydir Kovpak's partisan detachment united into one unit, with Rudniev becoming commissar. He quickly gained respect among partisan fighters and remained Kovpak's "commissar" while their initially small group rose into a large well-organized formation raiding the rear of the Axis occupants. In the winter of 1942, Rudniev was wounded in battle near Vesyloye village. On April 9, 1943 Semyon Rudniev was promoted to the rank of major general.