William J. Seymour (1870–1922) was an African American minister.

Seymour developed a belief in glossolalia ("speaking in tongues") as a confirmation of the gifts of the Holy Spirit. As a consequence of teaching this, he was removed from the Los Angeles parish where he had formerly ministered. Looking for a place to continue his work, he found a run-down building in downtown Los Angeles located on Azusa Street, and preached his doctrinal beliefs there.

The result was the Azusa Street Revival. Seymour not only rejected the existing racial barriers in favor of "unity in Christ", he also rejected the then almost-universal barriers to women in any form of church leadership. This revival meeting extended from 1906 until 1909, and became the subject of intense investigation by more mainstream Protestants. Some left feeling that Seymour's views were heresy, while others accepted his teachings and returned to their own congregations to expound them. The resulting movement became widely known as "Pentecostalism", likening it to the mainfestations of the Holy Spirit recorded as occurring in the first two chapters of Acts as occurring from ten days prior to the Feast of Pentecost up to that day.

Most of the current charismatic groups can claim some lineage linking them to the Azusa Street Revival and William Seymour. While the movement was largely to fracture along racial lines within a decade, the splits were in some ways perhaps less deep that the vast divide that seems often to divide many white religious denominations from their black counterparts. Probably the deepest split in the Pentecostal movement today is not racial, but rather between Trinitarian and "Jesus Only" theologies.

While there had been similar manifestations in the past (the Cane Ridge, Kentucky revival a century before in the Second Great Awakening being one such example), the current worldwide Pentecostal and charismatic movements are generally agreed to have been outgrowths of Seymour's ministry and the Azusa Street Revival. A play commemorating Seymour and the revival, Miracle on Azusa Street, is sometimes produced by Pentecostal churches both to teach their own members about their religious origins and as an outreach to those outside.