Frederick I (Fredrik I; 28 April 1676 – 5 April 1751) was King of Sweden from 1720 until his death, having been prince consort of Sweden from 1718 to 1720, and was also Landgrave of Hesse-Kassel from 1730. He ascended the throne following the death of his brother-in-law absolutist Charles XII in the Great Northern War, and the abdication of his wife, Charles's sister and successor Ulrika Eleonora, after she had to relinquish most powers to the Riksdag of the Estates and thus chose to abdicate. His powerless reign and lack of legitimate heirs of his own saw his family's elimination from the line of succession after the parliamentary government dominated by pro-revanchist Hat Party politicians ventured into a war with Russia, which ended in defeat and the Russian tsarina Elizabeth getting Adolf Frederick of Holstein-Gottorp instated following the death of the king. Whilst being the only Swedish monarch called Frederick, he was Frederick I of Hesse-Kassel and thus Frederick I also of Sweden, though other Swedish monarchs with non-repeating names (such as Birger, Sigismund and Frederick's successor: Adolf Frederick) have not been given numerals.
He was the son of Charles I, Landgrave of Hesse-Kassel, and Princess Maria Amalia of Courland. In 1692 the young prince made his Grand Tour to the Dutch Republic, in 1695 to the Italian Peninsula and later he studied in Geneva. After this he had a military career, leading the Hessian troops as Lieutenant General in the War of the Spanish Succession on the side of the Dutch. He was defeated in 1703 in the Battle of Speyerbach, but participated the next year in the great victory in the Battle of Blenheim. In 1706 he was again defeated by the French in the Battle of Castiglione. Both in 1716 and 1718 he joined the campaign of Charles XII of Sweden against Norway, and was appointed Swedish Generalissimus.
He married his second wife, Princess Ulrika Eleonora of Sweden, in 1715. He was then granted the title Prince of Sweden, with the style Royal Highness, by the estates, and was prince consort there during Ulrika Eleonora's rule as queen regnant from 1718 until her abdication in 1720.