Mary Dyer was an English and colonial American Puritan-turned-Quaker who was hanged in Boston, Massachusetts Bay Colony, for repeatedly defying a Puritan law banning Quakers from the colony due to their theological expansion of the Puritan concept of a church of individuals regenerated by the Holy Spirit to the idea of the indwelling of the Spirit or the "Light of Christ", which was deemed dangerous heresy. She is one of the four executed Quakers known as the Boston martyrs.
Dyer's birthplace has not been established, but it is known that she was married in London in 1633 to William Dyer, a member of the Fishmongers' Company but a milliner by profession. Mary and William were Puritans who were interested in reforming the Anglican Church from within, without separating from it.