Albert of Prussia (German: Albrecht von Preussen; 17 May 1490 – 20 March 1568) was a German prince who was the 37th grand master of the Teutonic Knights and, after converting to Lutheranism, became the first ruler of the Duchy of Prussia, the secularized state that emerged from the former Monastic State of the Teutonic Knights. Albert was the first European ruler to establish Lutheranism, and thus Protestantism, as the official state religion of his lands. He proved instrumental in the political spread of Protestantism in its early stage, ruling the Prussian lands for nearly six decades (1510–1568).

Albert was great-grandson of the converted pagan ruler Jogaila of Poland and Lithuania, vanquisher of the Teutonic Knights at the Battle of Grunwald. He was also a member of the Brandenburg-Ansbach branch of the House of Hohenzollern. He became grand master of the Teutonic Knights in their attempt to diplomatically win over the Polish-Lithuanian union. His skill in political administration and leadership ultimately succeeded in reversing the decline of the Teutonic Order. But Albert was sympathetic to the demands of Martin Luther, whose teachings had become popular in his lands. So he rebelled against the Roman Catholic Church and the Holy Roman Empire by converting the Teutonic state into a Protestant and hereditary realm, the Duchy of Prussia, for which he paid homage to his uncle, Sigismund I, king of Poland. That arrangement was confirmed by the Treaty of Kraków in 1525. Albert pledged a personal oath to the king and in return was invested with the duchy for himself and his heirs.

Albert's rule in Prussia was fairly prosperous. Although he had some trouble with the peasantry, the confiscation of the lands and treasures of the Catholic Church enabled him to propitiate the nobles and provide for the expenses of the newly established Prussian court. He was active in imperial politics, joining the League of Torgau in 1526, and acted in unison with the Protestants in plotting to overthrow Emperor Charles V after the issue of the Augsburg Interim in May 1548. Albert established schools in every town and founded the University of Königsberg in 1544. He promoted culture and arts, patronising the works of Erasmus Reinhold and Caspar Hennenberger. During the final years of his rule, Albert was forced to raise taxes instead of further confiscating now-depleted church lands, causing peasant rebellion. The intrigues of the court favourites Johann Funck and Paul Skalić also led to various religious and political disputes. Albert spent his final years virtually deprived of power, and died at Tapiau on 20 March 1568. His son, Albert Frederick, succeeded him as Duke of Prussia.

Albert was born in Ansbach in Franconia as the third son of Frederick I, Margrave of Brandenburg-Ansbach. His mother was Sophia, daughter of Casimir IV Jagiellon, Grand Duke of Lithuania and king of Poland, and his wife Elisabeth of Austria. His great-grandfather was Władysław II Jagiełło, the last pagan ruler in Europe, who, after converting to Christianity, defeated the Teutonic Knights at the Battle of Grunwald in 1410. He was raised for a career in the Church and spent some time at the court of Hermann IV of Hesse, Elector of Cologne, who appointed him canon of the Cologne Cathedral. Not only was he quite religious; he was also interested in mathematics and science and sometimes is claimed to have contradicted the teachings of the Church in favour of scientific theories. His career was forwarded by the Church, however, and institutions of the Catholic clerics supported his early advancement.

Turning to a more active life, Albert accompanied Emperor Maximilian I to Italy in 1508 and after his return spent some time in the Kingdom of Hungary.

Duke Frederick of Saxony, grand master of the Teutonic Order, died in December 1510. Albert was chosen as his successor early in 1511 in the hope that his relationship to his maternal uncle, Sigismund I the Old, Grand Duke of Lithuania and king of Poland, would facilitate a settlement of the disputes over eastern Prussia, which had been held by the order under Polish suzerainty since the Second Peace of Thorn (1466).

The new grand master, aware of his duties to the empire and to the papacy, refused to submit to the crown of Poland. As war over the order's existence appeared inevitable, Albert made strenuous efforts to secure allies and carried on protracted negotiations with Emperor Maximilian I. The ill-feeling, influenced by the ravages of members of the Order in Poland, culminated in a war which began in December 1519 and devastated Prussia. Albert was granted a four-year truce early in 1521.

The dispute was referred to Emperor Charles V and other princes, but as no settlement was reached Albert continued his efforts to obtain help in view of a renewal of the war. For this purpose, he visited the Diet of Nuremberg in 1522, where he made the acquaintance of the Reformer Andreas Osiander, by whose influence Albert was won over to Protestantism.