Joseph Jenckes Jr. (baptized October 12, 1628 – January 4, 1717), also spelled Jencks and Jenks, was the founder of Pawtucket, Rhode Island, where he erected a forge in 1671.

After his mother and only sibling died in England, his father, Joseph Jenckes Sr., immigrated to New England. A few years later, in about 1647, Jenckes Jr. joined his father at his forge in Massachusetts Bay Colony and learned his father's trade. In 1661, Jenckes was jailed for treason, a charge that was later dropped.

Jenckes moved to the Colony of Rhode Island and Providence Plantations where he became a successful businessman. He served many years in the Rhode Island General Assembly and was elected Speaker of the House. His son, Joseph, became the colony's governor.

Joseph Jenckes Jr. was baptized October 12, 1628, in Colnbrook, Buckinghamshire, England. He was the eldest of two children born to Joseph Jenckes Sr. (1599–1683) and Joan Hearne (1607–1635).

In his youth he lived in Hounslow, Middlesex, where his father worked as a cutler in a sword factory. His mother died in 1635 and his only sibling, Elizabeth, died in 1638. About 1642, the widower Joseph Jenckes Sr. immigrated to New England and by 1645 he was working to establish an iron works, later called the Saugus Iron Works, at Hammersmith near Lynn in Massachusetts Bay Colony. By 1647, Jenckes Jr., who had remained in England, joined his father at the iron works.

Sometime before 1660, after working with his father at the Saugus Iron Works, Jenckes moved to Concord to work at an iron smelting operation. When he returned to Lynn it was alleged that he made treasonous remarks in the Anchor Tavern against King Charles II of England who was to be crowned on April 23, 1661. Jenckes was arrested and imprisoned. During his hearing on April 1, 1661, he was accused by Nicholas Pinion of saying that "if he hade the King heir, he wold cutte off his head and make a football of it" and by Thomas Tower of saying "I should rather that his head were as his father's rather than he should come to England to set up popery there," an allusion to the 1649 beheading of Charles I. After seven weeks in prison, on May 22, 1661, the General Court of Massachusetts Bay Colony ruled in Jenckes's favor, citing his subsequent statement supporting the king. The charges were dropped and he was released. The decision was recorded as follows:

Joseph Jencks, Juñ, being accused & bound ouer to this Court for high misdemeano^r in diuers treasonable words ag^t the kings maj^ty, w^ch, vpon examination, he vtterly disounes, neither doeth it appeare that the same cann be legally prooved a^st him, only in part, for w^ch he presenteth & pleadeth the kings gracious act of indempnity, this Court therefore dischardgeth him from his imprisonm̃t.

Sometime between 1661 and 1669, Joseph Jenckes Jr. moved to the Colony of Rhode Island and Providence Plantations. In 1669, he was granted timbered land on both sides of the Pawtuxet River in Pawtuxet—then southern Providence—where he erected a sawmill. His grant required him to provide lumber and timber rights to the proprietors.