William Penn (24 October  [O.S. 14 October]  1644 – 10 August [O.S. 30 July 1718]  1718) was an English writer, religious thinker, and influential Quaker who founded the Province of Pennsylvania during the British colonial era. Penn was an advocate of democracy and religious freedom known for his amicable relations and successful treaties with the Lenape Native Americans who had resided in present-day Pennsylvania prior to European settlements in the state.

In 1681, King Charles II granted a large piece of his North American land holdings along the North Atlantic Ocean coast to Penn to offset debts he owed Penn's father, the admiral and politician Sir William Penn. The land included the present-day states of Pennsylvania and Delaware. The following year, in 1682, Penn left England for what was then British America, sailing up Delaware Bay and the Delaware River past earlier Swedish and Dutch riverfront colonies in what is present-day New Castle, Delaware. On this occasion, the colonists pledged allegiance to Penn as their new proprietor, and the first Pennsylvania General Assembly was held.

Penn then journeyed further north up the Delaware River and founded Philadelphia on the river's western bank. Penn's Quaker government was not viewed favorably by the previous Dutch and Swedish colonists, and earlier English settlers in what is now Delaware and was claimed for half a century by the neighboring Province of Maryland's proprietor family, the Calverts and Lord Baltimore. These earlier colonists had no historical allegiance to Pennsylvania, and almost immediately began petitioning for their own representative assembly. Twenty-three years later, in 1704, they achieved their goal when the three southernmost counties of provincial Pennsylvania along the western coast of the Delaware River were granted permission to split off and become the new semi-autonomous Delaware Colony. As the most prominent, prosperous and influential settlement in the new colony, New Castle, the original Swedish colony town became the colony's first capital.

As one of the earlier supporters of colonial unification, Penn wrote and urged for a union of all the English colonies in what, following the American Revolutionary War, later became the United States. The democratic principles that he included in the West Jersey Concessions and set forth in the Pennsylvania Frame of Government inspired delegates to the Constitutional Convention in Philadelphia to frame the U.S. Constitution, which was ratified by the delegates in 1787.

A man of deep religious conviction, Penn authored numerous works, exhorting believers to adhere to the spirit of Primitive Christianity. Penn was imprisoned several times in the Tower of London due to his faith, and his book No Cross, No Crown, published in 1669, which he authored from jail, has become a classic of Christian theological literature.