Nils Svante Flyg (9 June 1891 – 9 January 1943) was a Swedish Communist politician who turned pro-Nazi during World War II.
Nils Flyg was born and raised in Södermalm, a working-class area of . Early on he joined the Swedish Social Democratic Party's youth organization, the Swedish Social Democratic Youth League. In 1917, Flyg took part in the founding of a new leftist party, a group headed by Zeth Höglund and Karl Kilbom, which would soon become the Communist Party of Sweden.
Flyg became an important leader of the Communist Party, wrote books and went on political trips to the Soviet Union. In the general election of 1928, with the Flyg-dominated Communists cooperating with the dominant Social Democratic Party, he failed to achieve an influential position as voters failed to show substantial support for a Communist-Social Democratic coalition. In 1929 Flyg, along with the majority of the party's membership, was accused of insufficient loyalty to the Soviet-dominated Comintern and expelled from the party. The same year Flyg and Kilbom founded a new, parallel Communist Party, which claimed to be the real Communist Party of Sweden.
Initially Flyg and Kilbom attempted to reconcile with the Comintern, something that soon proved fruitless. They gradually developed an animosity towards Stalinism. By 1934 the party had changed name to the Socialist Party (Socialistiska partiet). At first, the Socialist Party still supported the Soviet Union but condemned the Stalinist leadership. But by the end of the 1930s, the party had changed its view and criticized the whole of the Soviet Union, a stance that gradually developed to a foreign policy embracing Nazi Germany.