Frederick I., first king of Prussia, son of the preceding, born in Konigsberg, July 22, 1657, died Feb. 25,1713. He became heir apparent on the death of his elder brother. Deformed by having been dropped from the arms of his nurse, and of weak constitution, his education was neglected, and thus his stepmother could the more easily persuade the old elector to bequeath a part of his possessions to her children. But Frederick, who was no less ambitious than his father, and was assured of the favor of the emperor Leopold I., on his accession as elector in 1688 under the name of Frederick III., took immediate possession of the whole inheritance, declaring the will null, and satisfying his step-brothers with offices and pensions. While vying in brilliancy with the court of Louis XIV., he also strenuously continued his father's policy of aggrandizement. Seeking the alliance of influential princes, he lent several of them his troops, on condition of mutual support or payment in money. Thus 6,000 of his soldiers aided William of Orange to secure the throne of England, and fought in the great battle of the Boyne; 20,000 fought successfully against the French, who had ravaged the Palatinate (1689); 15,000 joined the quadruple alliance of the Empire, Spain, Holland, and England, and fought on the Rhine (1690); 6,000 were sent (1691) to assist the emperor in his Hungarian war against the Turks, and contributed to the victories of Zalankemen, Belgrade, and Zenta. But all these services procured Frederick in the peace of Ryswick (1697) politically only the confirmation of the stipulations granted to his father by the treaties of Westphalia and St. Germain. Private negotiations, however, with several reigning houses gave him in part the immediate possession of, and in part hereditary claims to, various territories, which greatly enlarged the limits of his dominions.

He gained the royal crown only after long negotiations by a treaty with the emperor, concluded Nov. 16, 1700, and based on the humiliating obligation to aid the emperor with 10,000 troops in the threatening war of the Spanish succession, to support the house of Austria in every debate in the diet, and to vote for its princes at every imperial election. Hastening to Konigsberg in the midst of winter, Frederick placed the crown on his own head and on that of his wife, the sister of George I. of England, Jan. 18, 1701. On this occasion he founded the order of the black eagle. In the wars of Charles XII. of Sweden Frederick took no part, being actively engaged in the support of his ally the emperor in the long struggle against Louis XIV. He sent to the army on the Danube 20,000 men, who took part in the battle of Blenheim (1704), and to Italy 6,000, who greatly contributed to Eugene's victory at Turin (1706). Frederick is praised for his natural kindness, love of his subjects, and loyalty to his allies; but his vanity, love of pomp, and extravagance led to ruinous extortions. He founded the university of Halle, the Berlin academies of science and of sculpture and painting, and the supreme court of appeal.

Like his father he defended Protestantism in Germany.