Applying the international legal framework governing the use of nuclear weapons to the facts of the war in Ukraine leads to a clear answer to the question of whether the use of nuclear weapons in Ukraine by Russia would be legal. While the 1996 International Court of Justice Advisory Opinion on the Threat or Use of Nuclear Weapons did not conclusively decide whether the use of nuclear weapons was per se illegal, by applying the legal framework articulated by the International Court of Justice to the facts of Russia’s war against Ukraine it is clear that any conceivable use of nuclear weapons in Ukraine by Russia would be illegal. This article builds on the holdings of the International Court of Justice advisory opinion by applying the fundamental principles of international humanitarian law to the war in Ukraine. This includes an evaluation of several types of potential Russian nuclear strikes, including a tactical nuclear strike on a city, at sea, or in a remote location of the battlefield. This analysis demonstrates that there is no Russian nuclear strike against Ukrainian targets that could satisfy the legal requirements of international humanitarian law. Therefore, the article advises Russian military commanders to refuse to launch any nuclear launch order against Ukraine in the present armed conflict.