Ivan IV Vasilyevich (Russian: Ива́н Четвёртый, Васи́льевич​), known in English as Ivan the Terrible (Russian: Ива́н Гро́зный​) (25 August 1530, Moscow – 28 March [O.S. 18 March] 1584, Moscow) was Grand Prince of Moscow from 1533. The epithet "Grozny" is associated with might, power and strictness, rather than horror or cruelty. Ivan oversaw numerous changes in the transition from a medieval nation state to an empire and emerging regional power, and became the first Tsar of a new and more powerful nation.

Historic sources present disparate accounts of Ivan: he is described as intelligent and devout by some; given to rages and prone to episodic outbreaks of mental illness by others. One notable outburst resulted in the death of his groomed and chosen heir Ivan Ivanovich (although this version is supported mainly by foreign authors of that time and those Russian historians who quote them, the exact cause of his death is still disputed), and resulted in the passing of the Tsardom to the younger son: the weak and possibly mentally retarded Feodor I of Russia. His long reign saw the conquest of the Khanates of Kazan, Astrakhan, and Siberia, transforming Russia into a multiethnic and multiconfessional state spanning almost 1 billion acres, growing during his term at a rate of approximately 130 square kilometers a day.

Ivan was the son of Vasili III, who had divorced his first wife in 1525 on the grounds that she was barren (he charged her with sorcery and had her forcibly tonsured as a nun). When Ivan was just three years old his father died from a boil and inflammation on his leg which developed into blood poisoning. Ivan was proclaimed the Grand Prince of Moscow at his father's request. At first, his mother Elena Glinskaya acted as a regent, but she died of what many believe to be assassination via poison when Ivan was only eight years old. She was replaced as regent by boyars from the Shuisky family until Ivan assumed power in 1544. According to his own letters, Ivan and his younger brother Yuri customarily felt neglected and offended by the mighty boyars from the Shuisky and Belsky families.

Ivan was crowned tsar with Monomakh's Cap at the Cathedral of the Dormition at age sixteen on 16 January 1547. Despite calamities triggered by the Great Fire of 1547, the early part of his reign was one of peaceful reforms and modernization. Ivan revised the law code (known as the sudebnik), created a standing army (the streltsy), established the Zemsky Sobor or assembly of the land, a public, consensus-building assembly, the council of the nobles (known as the Chosen Council), and confirmed the position of the Church with the Council of the Hundred Chapters, which unified the rituals and ecclesiastical regulations of the entire country. He introduced local self-government to rural regions, mainly in the northeast of Russia, populated by the state peasantry. During his reign the first printing press was introduced to Russia (although the first Russian printers Ivan Fedorov and Pyotr Mstislavets had to flee from Moscow to the Grand Duchy of Lithuania). In 1547 Hans Schlitte, the agent of Ivan, employed craftsmen in Germany for work in Russia. However all these craftsmen were arrested in Lübeck at the request of Poland and Livonia. The German merchant companies ignored the new port built by Ivan on the river Narva in 1550 and continued to deliver goods in the Baltic ports owned by Livonia. Russia remained isolated from sea trade.

Ivan formed new trading connections, opening up the White Sea and the port of Arkhangelsk to the Muscovy Company of English merchants. In 1552 he defeated the Kazan Khanate, whose armies had repeatedly devastated the Northeast of Russia, and annexed its territory. In 1556, he annexed the Astrakhan Khanate and destroyed the largest slave market on the river Volga. These conquests complicated the migration of the aggressive nomadic hordes from Asia to Europe through Volga and transformed Russia into a multinational and multiconfessional state.

Ivan IV corresponded with Orthodox leaders overseas as well. In response to a letter of Patriarch Joachim of Alexandria asking the Tsar for financial assistance for the Monastery of St. Catherine in Sinai, which had suffered from the Turks, Ivan IV sent in 1558 a delegation to Egypt lead by archdeacon Gennady, who, however, died in Constantinople before he could reach Egypt. From then on, the embassy was headed by a Smolensk merchant Vasily Poznyakov. Poznyakov's delegation visited Alexandria, Cairo, and Sinai, brought the patriarch a fur coat and an icon sent by the Tsar, and left an interesting account of its two and half years' travels.

The Tsar had St. Basil's Cathedral constructed in Moscow to commemorate the seizure of Kazan. Legend has it that he was so impressed with the structure that he had the architect, Postnik Yakovlev, blinded, so that he could never design anything as beautiful again. In fact, it is known that Yakovlev designed several churches and the kremlin walls in Kazan itself in the early 1560s, as well as the chapel over St. Vasilii's grave that was added to St. Basil's Cathedral in 1588, several years after Ivan's death, indicating that he had not, in fact, been blinded by the tsar years earlier.

Other events of this period include the introduction of the first laws restricting the mobility of the peasants, which would eventually lead to serfdom, and change in Ivan's personality, traditionally linked to his near-fatal illness in 1553 and the death of his first wife, Anastasia Romanovna in 1560. Ivan suspected boyars of poisoning his wife and of plotting to replace him on the throne with his cousin, Vladimir of Staritsa. In addition, during that illness Ivan had asked the boyars to swear an oath of allegiance to his eldest son, an infant at the time. Many boyars refused, deeming the tsar's health too hopeless for him to survive. This angered Ivan and added to his distrust of the boyars. There followed brutal reprisals and assassinations, including those of Metropolitan Philip and Prince Alexander Gorbatyi-Shuisky.

The 1565 formation of the Oprichnina was also significant. The Oprichnina was the section of Russia (mainly the Northeast) directly ruled by Ivan and policed by his personal servicemen, the Oprichniki. This system of Oprichnina has been viewed by some historians as a tool against the omnipotent hereditary nobility of Russia (boyars) who opposed the absolutist drive of the tsar, while others have interpreted it as a sign of the paranoia and mental deterioration of the tsar. Additionally, Ivan had made overtures to Queen Elizabeth I of England several times toward the end of his reign, inquiring about the possibility of fleeing Moscow and being granted asylum in her realm; this also has been interpreted by some as another possible sign of his deteriorating mental health.

The later half of Ivan's reign was less successful. Although Khan Devlet I Giray of Crimea repeatedly devastated the Moscow region and even set Moscow on fire in 1571, the Tsar supported Yermak's conquest of Tatar Siberia, adopting a policy of empire-building, which led him to launch a victorious war of seaward expansion to the west, only to find himself fighting the Swedes, Lithuanians, Poles, and the Livonian Teutonic Knights. For twenty-four years the Livonian War dragged on, damaging the Russian economy and military and failing to gain any territory for Russia. In the 1560s the combination of drought and famine, Polish-Lithuanian raids, Tatar invasions, and the sea-trading blockade carried out by the Swedes, Poles and the Hanseatic League devastated Russia. The price of grain increased by a factor of ten. Epidemics of the plague killed 10,000 in Novgorod. In 1570 the plague killed 600-1000 in Moscow daily. One of Ivan's advisors, Prince Andrei Kurbsky, defected to the Lithuanians, headed the Lithuanian troops and devastated the Russian region of Velikiye Luki. This treachery deeply hurt Ivan. As the Oprichnina continued, Ivan became mentally unstable and physically disabled. In one week, he could easily pass from the most depraved orgies to anguished prayers and fasting in a remote northern monastery.

Because he gradually grew unbalanced and violent, the Oprichniks under Malyuta Skuratov soon got out of hand and became murderous thugs. They massacred nobles and peasants, and conscripted men to fight the war in Livonia. Depopulation and famine ensued. What had been by far the richest area of Russia became the poorest. In a dispute with the wealthy city of Novgorod, Ivan ordered the Oprichniks to murder inhabitants of this city, which was never to regain its former prosperity. His followers burned and pillaged the city and villages. As many as 60,000 may have been killed during the infamous Massacre of Novgorod in 1570; many others were deported elsewhere. Yet the official death toll named 1,500 of Novgorod bigpeople (nobility) and mentioned only about the same number of smaller people. Many modern researchers estimate the number of victims between two and three thousand. (After the famine and epidemics of 1560s the population of Novgorod perhaps did not exceed 10,000-20,000.)

Having rejected peace proposals from his enemies, Ivan IV found himself in a difficult position by 1579, when the Crimean Khanate devastated Muscovian territories and even burnt down Moscow (Russo-Crimean Wars). The dislocations in population fleeing the war compounded the effects of the concurrently occurring drought and exacerbated war engendered epidemics causing much loss of population. All together, the prolonged war had nearly fatally affected the economy, Oprichnina had thoroughly disrupted the government, while The Grand Principality of Lithuania had united with The Kingdom of Poland and acquired an energetic leader, Stefan Batory, who was supported by Russia's southern enemy, The Ottoman Empire (1576). Ivan's realm was now squeezed by two of the great powers of the day.

With the failure of negotiations, Batory replied with a series of three offensives against Muscovy in each campaign seasons of 1579–1581, trying to cut The Kingdom of Livonia from Muscovian territories. During his first offensive in 1579, he retook Polotsk with 22,000 men. During the second, in 1580, he took Velikie Luki with a 29,000-strong army. Finally, he started the Siege of Pskov in 1581 with a 100,000-strong army. Frederick II had trouble continuing the fight against Muscovy, unlike Sweden and Poland. He came to an agreement with John IIIin 1580 giving him the titles in Livonia. That war would last from 1577 to 1582. Muscovy recognized Polish-Lithuanian control of Ducatus Ultradunensis only in 1582. After Magnus von Lyffland died in 1583, Poland invaded his territories in The Duchy of Courland and Frederick II decided to sell his rights of inheritance. Except for the island of Saaremaa, Denmark was out of the Baltic by 1585. As of 1598, Polish Livonia was divided into:

• Wenden Voivodeship (województwo wendeńskie, Kieś)
• Dorpat Voivodeship (województwo dorpackie, Dorpat)
• Parnawa Voivodeship (województwo parnawskie, Parnawa)

In 1581, Ivan beat his pregnant daughter-in-law for wearing immodest clothing, and this may have caused a miscarriage. His son, also named Ivan, upon learning of this, engaged in a heated argument with his father, which resulted in Ivan striking his son in the head with his pointed staff, causing his son's (accidental) death. This event is depicted in the famous painting by Ilya Repin, Ivan the Terrible and his son Ivan on Friday, 16 November 1581 better known as Ivan the Terrible killing his son.

Ivan died perhaps while setting up a chess board brought to him by Bogdan Belsky awaiting the arrival of Boris Godunov to play or else while actually playing chess with Belsky on 18 March 1584. Upon Ivan's death, the ravaged kingdom was left to his unfit and childless son Feodor.

Today, there exists a controversial movement in Russia campaigning in favor of granting sainthood to Ivan IV. The Russian Orthodox Church has stated their opposition to the idea.

D.S. Mirsky called Ivan "a pamphleteer of genius". The epistles attributed to him are the masterpieces of old Russian (perhaps all Russian) political journalism. They may be too full of texts from the Scriptures and the Fathers, and their Church Slavonic is not always correct. But they are full of cruel irony, expressed in pointedly forcible terms. The shameless bully and the great polemicist are seen together in a flash when he taunts the runaway prince Kurbsky with the question: "If you are so sure of your righteousness, why did you run away and not prefer martyrdom at my hands?" Such strokes were well calculated to drive his correspondent into a rage. "The part of the cruel tyrant elaborately upbraiding an escaped victim while he continues torturing those in his reach may be detestable, but Ivan plays it with truly Shakespearian breadth of imagination". These letters are often the only existing source on Ivan's personality and provide crucial information on his reign, but Harvard professor Edward Keenan has argued that these letters are 17th century forgeries. This contention, however, has not been widely accepted, and other scholars, such as John Fennell and Ruslan Skrynnikov continued to argue for their authenticity. Recent archival discoveries of 16th century copies of the letters strengthen the argument for their authenticity. Besides his letters to Kurbsky he wrote other satirical invectives to men in his power. The best is his letter to the abbot of the Kirillo-Belozersky Monastery, where he pours out all the poison of his grim irony on the unascetic life of the boyars, shorn monks, and those exiled by his order. His picture of their luxurious life in the citadel of ascetism is a masterpiece of trenchant sarcasm.