The Second World War began in September 1939, when Hitler and Stalin invaded Poland, the Soviet Union taking most of Eastern Poland. Nazi Germany with its allies invaded the Soviet Union in 1941. Some Ukrainians initially regarded the Wehrmacht soldiers as liberators from Soviet rule, while others formed a partisan movement. Some elements of the Ukrainian nationalist underground formed a Ukrainian Insurgent Army that fought both Soviet forces and the Nazis. Others collaborated with the Germans. In Volhynia, Ukrainian fighters committed a massacre against up to 100,000 Polish civilians. Residual small groups of the UPA-partizans acted near the Polish and Soviet border as long as to the 1950s. Galicia, Volhynia, South Bessarabia, Northern Bukovina, and Carpathian Ruthenia were added as a result of the Molotov–Ribbentrop Pact in 1939 and the Soviet victory over Germany in the Second World War, 1939–45.

After World War II, some amendments to the Constitution of the Ukrainian SSR were accepted, which allowed it to act as a separate subject of international law in some cases and to a certain extent, remaining a part of the Soviet Union at the same time. In particular, these amendments allowed the Ukrainian SSR to become one of the founding members of the United Nations (UN) together with the Soviet Union and the Byelorussian SSR. This was part of a deal with the United States to ensure a degree of balance in the General Assembly, which, the USSR opined, was unbalanced in favor of the Western Bloc. In its capacity as a member of the UN, the Ukrainian SSR was an elected member of the United Nations Security Council in 1948–1949 and 1984–1985. The Crimean Oblast was transferred from the RSFSR to the Ukrainian SSR in 1954.