Ukraine is a country in Eastern Europe. It is the second-largest country in Europe after Russia, which borders it to the east and northeast. Ukraine also borders Belarus to the north; Poland and Slovakia to the west; Hungary, Romania and Moldova to the southwest; and the Black Sea and the Sea of Azov to the south and southeast. Kyiv is the nation's capital and largest city, followed by Kharkiv, Odesa, and Dnipro. Ukraine's official language is Ukrainian.

Humans have inhabited Ukraine since 32,000 BC. During the Middle Ages, it was the site of early Slavic expansion and later became a key centre of East Slavic culture under the state of Kievan Rus', which emerged in the 9th century. Kievan Rus' became the largest and most powerful realm in Europe in the 10th and 11th centuries, but gradually disintegrated into rival regional powers before being destroyed by the Mongols in the 13th century. For the next 600 years the area was contested, divided, and ruled by a variety of external powers including the Grand Duchy of Lithuania, the Kingdom of Poland, the Polish–Lithuanian Commonwealth, the Austrian Empire, the Ottoman Empire, and the Tsardom of Russia.

The Cossack Hetmanate emerged in central Ukraine in the 17th century but was partitioned between Russia and Poland before being absorbed by the Russian Empire in the late 19th century. Ukrainian nationalism developed and, following the Russian Revolution in 1917, the short-lived Ukrainian People's Republic was formed. The Bolsheviks consolidated control over much of the former empire and established the Ukrainian Soviet Socialist Republic, which became a constituent republic of the Soviet Union in 1922. In the early 1930s, millions of Ukrainians died in the Holodomor, a human-made famine. During World War II, Ukraine was occupied by Germany and endured major battles and atrocities, resulting in 7 million civilians killed, including most Ukrainian Jews.

Ukraine gained independence in 1991 as the Soviet Union dissolved and declared itself neutral. A new constitution was adopted in 1996 as the country transitioned to a free market liberal democracy amid endemic corruption and a legacy of state control. The Orange Revolution of 2004–2005 ushered electoral and constitutional reforms. Resurgent political crises prompted a series of mass demonstrations in 2014 known as the Euromaidan, leading to a revolution, at the end of which Russia unilaterally occupied and annexed Ukraine's Crimean Peninsula, and pro-Russian unrest culminated in a war in Donbas with Russian-backed separatists and Russia. Russia launched a full-scale invasion of Ukraine in 2022.

Ukraine is a unitary state and its system of government is a semi-presidential republic. Ukraine has a transition economy and has the lowest nominal GDP per capita in Europe as of 2024, with corruption being a significant issue. Due to its extensive fertile land, the country is an important exporter of grain, with grain produce reduced since 2022 due to the Russian invasion, endangering global food security. Ukraine is considered a middle power in global affairs. Its military is the sixth largest in the world with the eighth largest defence budget, and operates one of the world's largest and most diverse drone fleets. Ukraine is a founding member of the United Nations and a member of the Council of Europe, the World Trade Organisation, and the OSCE. It has been in the process of joining the European Union and applied to join NATO in 2022.

On 24 February 2022, Russia invaded Ukraine, starting the largest and deadliest war in Europe since World War II, in a major escalation of the conflict between the two countries which began in 2014. The fighting has caused hundreds of thousands of military casualties and tens of thousands of Ukrainian civilian casualties. As of 2025, Russian troops occupy about 20% of Ukraine. From a population of 41 million, about 8 million Ukrainians had been internally displaced and more than 8.2 million had fled the country by April 2023, creating Europe's largest refugee crisis since World War II.

In late 2021, Russia massed troops near Ukraine's borders and issued demands to the West including a ban on Ukraine ever joining the NATO military alliance. After repeatedly denying having plans to attack Ukraine, on 24 February 2022, Russian president Vladimir Putin announced a "special military operation", saying that it was to support the Russian-backed breakaway republics of Donetsk and Luhansk, whose paramilitary forces had been fighting Ukraine in the war in Donbas since 2014. Putin espoused irredentist and imperialist views challenging Ukraine's legitimacy as a state, baselessly claimed that the Ukrainian government were neo-Nazis committing genocide against the Russian minority in the Donbas, and said that Russia's goal was to "demilitarise and denazify" Ukraine. Russian air strikes and a ground invasion were launched on a northern front from Belarus towards the capital Kyiv, a southern front from Crimea, and an eastern front from the Donbas and towards Kharkiv. Ukraine enacted martial law, ordered a general mobilisation, and severed diplomatic relations with Russia.

Russian troops retreated from the north and the outskirts of Kyiv by April 2022, after encountering stiff resistance and logistical challenges. The Bucha massacre was uncovered after their withdrawal. In the southeast, Russia launched an offensive in the Donbas and captured Mariupol after a destructive siege. Russia continued to bomb military and civilian targets far from the front, and struck the energy grid during winter months. In late 2022, Ukraine launched successful counteroffensives in the south and east, liberating most of Kharkiv Oblast. Soon after, Russia illegally annexed four partly-occupied provinces. In November, Ukraine liberated Kherson. In June 2023, Ukraine launched another counteroffensive in the southeast but made few gains. After small but steady Russian advances in the east in the first half of 2024, Ukraine launched a cross-border offensive into Russia's Kursk Oblast in August, where North Korean soldiers were sent to assist Russia. The United Nations Human Rights Office reports that Russia is committing severe human rights violations in occupied Ukraine. The direct cost of the war for Russia has been over US$450 billion.

The invasion was met with widespread international condemnation. The United Nations General Assembly passed a resolution condemning the invasion and demanding a full Russian withdrawal. The International Court of Justice ordered Russia to halt military operations, and the Council of Europe expelled Russia. Many countries imposed sanctions on Russia and its ally Belarus and provided large-scale humanitarian and military aid to Ukraine. The Baltic states and Poland declared Russia a terrorist state. Protests occurred around the world, with anti-war protesters in Russia being met by mass arrests and greater media censorship. The Russian attacks on civilians have led to allegations of genocide. War-related disruption to Ukrainian agriculture and shipping contributed to a world food crisis, war-related local environmental damage has been described as ecocide and the war has heavily disrupted global climate policy. The International Criminal Court (ICC) opened an investigation into crimes against humanity, war crimes, abduction of Ukrainian children, and genocide against Ukrainians. The ICC issued arrest warrants for Putin and Maria Lvova-Belova and for four Russian military officials.