On Jan. 16, 1547, Ivan was crowned “tsar and grand prince of all Russia.” The title tsar was derived from the Latin title “caesar” and was translated by Ivan's contemporaries as “emperor.” In February 1547 Ivan married Anastasiya Romanovna, a great-aunt of the future first tsar of the Romanov dynasty.

Since 1542 Ivan had been greatly influenced by the views of the metropolitan of Moscow, Makari, who encouraged the young tsar in his desire to establish a Christian state based on the principles of justice. Ivan's government soon embarked on a wide program of reforms and of the reorganization of both central and local administration. Church councils summoned in 1547 and 1549 strengthened and systematized the church's affairs, affirming its Orthodoxy and canonizing a large number of Russian saints. In 1549 the first zemski sobor (zemsky sobor) was summoned to meet in an advisory capacity—this was a national assembly composed of boyars, clergy, and some elected representatives of the new service gentry. In 1550 a new, more detailed legal code was drawn up that replaced one dating from 1497. Russia's central administration was also reorganized into departments, each responsible for a specific function of the state. The conditions of military service were improved, the armed forces were reorganized, and the system of command altered so that commanders were appointed on merit rather than simply by virtue of their noble birth. The government also introduced extensive self-government, with district administrators elected by the local gentry.

One object of the reforms was to limit the powers of the hereditary aristocracy of princes and boyars (who held their estates on a hereditary basis) and promote the interests of the service gentry, who held their landed estates solely as compensation for service to the government and who were thus dependent on the tsar. Ivan apparently aimed at forming a class of landed gentry that would owe everything to the sovereign. All the reforms took place under the aegis of the so-called “Chosen Council,” an informal advisory body in which the leading figures were the tsar's favourites Aleksey Adashev and the priest Silvestr. The council's influence waned and then disappeared in the early 1560s, however, after the death of Ivan's first wife and of Makari, by which time Ivan's views and his entourage had changed. Ivan's first wife, Anastasiya, died in 1560, and only two male heirs by her, Ivan (b. 1554) and Fyodor (b. 1557), survived the rigors of medieval childhood.

Russia was at war for the greater part of Ivan's reign. Muscovite rulers had long feared incursions by the Tatars (Tatar), and in 1547–48 and 1549–50 unsuccessful campaigns were undertaken against the hostile khanate of Kazan, on the Volga River. In 1552, after lengthy preparations, the tsar set out for Kazan, and the Russian army then succeeded in taking the town by assault. In 1556 the khanate of Astrakhan, located at the mouth of the Volga, was annexed without a fight. From that moment onward, the Volga became a Russian river, and the trade route to the Caspian Sea was rendered safe.

With both banks of the Volga now secured, Ivan prepared for a campaign to force an exit to the sea, a traditional concern of landlocked Russia. Ivan felt that trade with Europe depended on free access to the Baltic and decided to turn his attention westward. In 1558 he went to war in an attempt to establish Russian rule over Livonia (in present-day Latvia and Estonia). Russia was at first victorious and succeeded in destroying the Livonian knights, but their ally Lithuania became an integral part of Poland in 1569. The war dragged on; while the Swedes supported Poland against Russia, the Crimean Tatars attacked Astrakhan and even made an extensive incursion into Russia in 1571; they burned Moscow, leaving only the Kremlin standing. When Stephen Báthory of Transylvania became king of Poland in 1575, reorganized Polish armies under his leadership were able to carry the war onto Russian territory while the Swedes recaptured parts of Livonia. Ivan at last asked Pope Gregory XIII to intervene, and through the mediation of his nuncio, Antonio Possevino, an armistice with Poland was concluded on Jan. 15, 1582. Under its terms Russia lost all its gains in Livonia, and an armistice with Sweden in 1583 compelled Russia to give up towns on the Gulf of Finland. The 24-year-long Livonian War had proved fruitless for Russia, which was exhausted by the long struggle.

Ivan's first executions apparently arose out of his disappointment over the course of the Livonian War and the suspected treason of several Russian boyars. The defection of one of Ivan's outstanding field commanders, Prince Andrey Kurbsky (Kurbsky, Andrey Mikhaylovich, Prince), to Poland in 1564 greatly startled the tsar, who announced later that year his intention of abdicating in view of the boyars' betrayal. The Muscovites, however, led by the clergy, implored him to continue to rule, and in 1565 he acceded to their request on condition that he should be allowed to deal with the traitors as he wished and that he should form an oprichnina—i.e., an aggregate of territory that would be administered separately from the rest of the state and put under his immediate control as crown land. A bodyguard of 1,000–6,000 men, known as the oprichniki, was raised; and specified towns and districts all over Russia were included in the oprichnina, their revenues being assigned to the maintenance of the tsar's new court and household, which consisted of a number of carefully selected boyars and service gentry. Ivan lived exclusively in this entourage and withdrew from the day-to-day management of Russia's administrative apparatus (now called the zemschina, or “the land”), which he left in the hands of leading boyars and bureaucrats. Ivan cut himself off from almost all communication with them, while the oprichniki trampled with impunity on everyone beyond Ivan's immediate circle.

Since nearly all the documents relating to this epoch were destroyed in one of Moscow's periodic fires, historians tend to give differing explanations for Ivan's actions during this part of his reign. The majority tend to the view that the struggle was between the tsar and the old hereditary nobility, which, jealous of surrendering its power and privileges, had resisted his internal reforms and military projects. The oprichnina thus may have been Ivan's attempt to create a highly centralized state and destroy the economic strength and political power of the princes and the high nobility. The increasingly resentful boyars had indeed opposed Ivan and plotted against him on occasion, but the reign of terror that Ivan initiated by the oprichnina proved far more dangerous to the stability of the country than the danger that it was designed to suppress. In 1570, for example, Ivan personally led his oprichniki troops against Novgorod, destroying that city and executing several thousand of its inhabitants. Many boyars and other members of the gentry perished during this period, some being publicly executed with calculated and symbolic cruelty. Ivan later sent to various monasteries memorials (sinodiki) of more than 3,000 of his victims, most of whom were executed in the course of the oprichnina.

The oprichnina lasted only seven years, from 1565 to 1572, when it was abolished as a result of the failure of the oprichnina regiments to defend Moscow from attack by the Crimean Tatars. The oprichnina army was reintegrated with that of the zemschina, and some of the estates confiscated by Ivan's followers were returned to their owners. The entire episode of the oprichnina leaves a bloody imprint on Ivan's reign, causing some doubts about his mental stability and leaving historians with the impression of a morbidly suspicious and vindictive ruler.

Withdrawal and flight are themes that run through the later years of Ivan's reign. He expressed an interest in establishing diplomatic and trade relations with England, even suggesting his readiness to marry an English noblewoman. In 1575 he seems to have abdicated for about a year in favour of a Tatar prince, Simeon Bekbulatovich. During the 1570s he married five wives in succession in only nine years. Finally, in a fit of rage, he murdered his only viable heir, Ivan, in 1581. This murder set the clock ticking for the political crisis, known as the Time of Troubles (Troubles, Time of), that began with the extinction of the Rurik dynasty upon the infirm Fyodor's death in 1598.

Ivan's achievements were many. In foreign policy all his actions were directed toward forcing Russia into Europe—a line that Peter I the Great was to continue. Internally, Ivan's reign of terror eventually resulted in the weakening of all levels of the aristocracy, including the service gentry he had sponsored. The prolonged and unsuccessful Livonian War overextended the state's resources and helped bring Russia to the verge of economic collapse. These factors, together with Tatar incursions, resulted in the depopulation of a number of Russian provinces by the time of Ivan's death in 1584. Nevertheless, he left his realm far more centralized both administratively and culturally than it had been previously.

Ivan also encouraged Russia's cultural development, especially through printing. He himself wrote well, and, though his surviving writings are mainly of a political nature, his command of words and his biting sarcasm are very evident. Ivan was a devout adherent of the Orthodox church. His arguments on religious questions are striking in their power and conviction, but he placed the most emphasis on defending the divine right of the ruler to unlimited power under God—a view with which most other monarchs of the time would have been in agreement.