"Köln" took part in Operation Weserübung, the invasion of Norway, in April 1940. She was assigned Group 3, tasked with the assault on Bergen, along with her sister "Königsberg". She reached the harbor unscathed, but "Königsberg" was not so lucky; she was badly damaged by Norwegian coastal guns. "Köln" nevertheless supported the German infantry ashore with her main guns. After the port was secured, she returned to Germany, along with a pair of destroyers. In late 1940, she went into drydock for further modifications. A degaussing coil was installed, along with a helicopter landing platform on top of turret "Bruno". She thereafter served as a testbed for the Flettner Fl 282 helicopter, a task she performed until 1942.