In 1827, Joseph revealed to family and friends that he had taken possession of the golden plates, a record of the ancient inhabitants of the Americas. He also claimed to possess an ancient breastplate and sword, and two mysterious crystals called the urim and thummim which he was using to translate the contents of the plates into English. Joseph refused to let anyone see the plates directly, though select individuals were allowed to touch the plates underneath a cloth or heft them inside of a box.

Joseph employed first his wife Emma, then a local wealthy farmer named Martin Harris, and finally a school teacher named Oliver Cowdery as his scribes to assist him in the translation of the plates. As word about the golden plates spread, Joseph attracted some curiosity seekers who came to believe in him, but he also began to be harrassed by locals who saw his stories about the plates as a kind of fraud similar to his old treasure-digging enterprises. Joseph and his family were also harried by break-ins, vandalism and trespassing, as locals ransacked his property in an effort to steal the plates. Joseph and Emma retreated to Harmony, to Emma's father's farm to avoid this harrassment.

The Book of Mormon purported to be the divinely inspired writings of ancient Americans, a New World analogue to the Bible. The book claimed, among other things, that Native Americans were actually the descendents of Hebrew refugees from the Babylonian captivity, who, with divine guidance, sailed across the ocean and colonized the Americas around 600 B.C. It also claimed that Jesus Christ appeared in the Americas after his resurrection in the Old World. The Book of Mormon also seemed to address many of the great religious controversies that were raging on the revivalist frontier at the time. In the course of translating the plates, Joseph also claimed to have received an angelic visit from John the Baptist, who conferred on him and on Oliver Cowdery the ancient "priesthood of Aaron," as a prelude to a continuing restoration of lost priesthoods and truths, and the reestablishment of God's one true church on the face of the earth.

In preparation for the publication of the Book of Mormon, Joseph Smith promised a number of his followers that they would be permitted to see and handle the golden plates, in order to authenticate them to the world. The testimonies of "the Three Witnesses" (Oliver Cowdery, David Whitmer and Martin Harris) and of the "Eight Witnesses" (Christian Whitmer, Jacob Whitmer, Peter Whitmer, Jr., John Whitmer, Hiram Page, Joseph Smith, Sr., Hyrum Smith and Samuel H. Smith) have ever since been published in the Book of Mormon right after the title page. At least one of the three witnesses (Martin Harris) later noted that they did not actually "see" the plates with their physical eyes, but with spiritual eyes. According to Joseph Smith, once the translation of the Book of Mormon was complete, the golden plates were retrieved by the Angel Moroni for safekeeping. Most Mormons have always accepted the existence of the golden plates on faith.