Following the joint German-Soviet invasion of Poland, which started World War II in September 1939, the town was occupied by Germany until 1945, and the population was subjected to various atrocities. In April 1940, the Germans arrested the pre-war Polish mayor Feliks Rostkowski, and imprisoned him in a concentration camp, however, he survived and returned to Nasielsk after the war. In December 1940, the Germans expelled around 1,000 Poles from the town. Expelled Poles were held in a camp in Działdowo for two weeks, where they were stripped of valuables, and then deported in freight trains to the Radom District of the General Government, while their houses and workshops were handed over to German colonists as part of the "Lebensraum" policy. From 1941 to 1943, the Germans operated a forced labour camp in the town.