Frederick William I (German: Friedrich Wilhelm I) (14 August 1688 – 31 May 1740) of the House of Hohenzollern, was the King in Prussia and Elector of Brandenburg (as Frederick William II) from 1713 until his death. He is popularly known as "the Soldier-King" (der Soldatenkönig). He was in personal union the sovereign prince of the Principality of Neuchâtel.

The King acquired a reputation for his fondness for military display, leading to his special efforts to hire the tallest men he could find in all of Europe for a special regiment nicknamed the Potsdam Giants.

He was born in Berlin to Frederick I of Prussia and Sophia Charlotte of Hanover. His father had successfully acquired the title King for the margraves of Brandenburg.

Frederick William's contributions to the state of Prussia primarily consisted of civil service reforms, developing the international reputation of the Prussian military, and increasing the overall efficiency and discipline of his military, which in turn placed Prussia as an entity on a par with Early Modern France, the Kingdom of Great Britain, and other politically dominant states in Europe during the 18th century.

During his reign, Frederick William I did much to centralize and improve Prussia. He replaced mandatory military service among the middle class with an annual tax, established primary schools, and resettled East Prussia (which had been devastated by the plague in 1709). Frederick William was an extremely able administrator. He opposed all superfluous spending, so long as it did not concern his army. Frederick William paid the consumer tax he himself had imposed, and no candles were left burning at court. He lived frugally and worked hard and tirelessly for the welfare of his people. He encouraged farming, reclaimed marshes, stored grain in good times and sold it in bad times. He dictated the manual of Regulations for State Officials, containing 35 chapters and 297 paragraphs in which every public servant in Prussia could find his duties precisely set out. A minister or councillor failing to attend a committee meeting would lose six months' pay. If he absented himself a second time, he would be discharged from the Royal service.

In short, Frederick William was extremely concerned by every little aspect of his relatively small country so that it suited all the needs, to defend itself. His rule was absolutist and he was a firm autocrat. He practiced rigid economy, never started a war, and at his death there was a large surplus in the treasury which was kept rather bizarrely in his basement. The Prussian army was made an efficient instrument. Although Frederick William built up one of the most powerful armies in Europe and loved military pomp, he was essentially a peaceful man. He intervened briefly in the Great Northern War, but gained little territory. The observation about the power of the pen being mightier than the sword has sometimes been attributed to him.

Though he was peaceful, he was by no means gentle. His eldest surviving son was Frederick II (Fritz), born in 1712. Frederick William wanted him to become a fine soldier. As a small child, Fritz was awakened each morning by the firing of a cannon. At the age of 6, he was given his own regiment of children to drill as cadets, and a year later, he was given a miniature arsenal. Fritz was beaten for being thrown off a bolting horse and wearing gloves in cold weather. Frederick William would frequently mistreat Fritz (he preferred his younger sibling August William). After the prince attempted to flee to England with his tutor, Hans Hermann von Katte, the father had Katte executed before the eyes of the prince, who himself was court-martialled. The court declared itself not competent in this case. Whether it was the king's intention to have his son executed as well (as Voltaire claims) is not clear. However, the Holy Roman Emperor Charles VI intervened, claiming that a prince could only be tried by the Reichstag itself. Frederick was imprisoned in the Fortress of Küstrin from 2 September to 19 November 1731 and exiled from court until February 1732.

Frederick William married his first cousin Sophia Dorothea of Hanover, George II's younger sister (daughter of his uncle, King George I of Great Britain and Sophia Dorothea of Celle) on 28 November 1706. They had fourteen children.