Pentecostalism or classical Pentecostalism is a Protestant Charismatic Christian movement that emphasizes direct personal experience of God through baptism with the Holy Spirit. The term Pentecostal is derived from Pentecost, an event that commemorates the descent of the Holy Spirit upon the Apostles and other followers of Jesus Christ while they were in Jerusalem celebrating the Feast of Weeks, as described in the Acts of the Apostles (Acts 2:1–31).

Like other forms of evangelical Protestantism, Pentecostalism adheres to the inerrancy of the Bible and the necessity of the New Birth: an individual repenting of their sin and "accepting Jesus Christ as personal Lord and Savior". It is distinguished by belief in the "baptism in the Holy Spirit" that enables a Christian to "live a Spirit-filled and empowered life". This empowerment includes the use of spiritual gifts: such as speaking in tongues and divine healing. Because of their commitment to biblical authority, spiritual gifts, and the miraculous, Pentecostals see their movement as reflecting the same kind of spiritual power and teachings that were found in the Apostolic Age of the Early Church. For this reason, some Pentecostals also use the term "Apostolic" or "Full Gospel" to describe their movement.

Holiness Pentecostalism emerged in the early 20th century among radical adherents of the Wesleyan-Holiness movement, who were energized by Christian revivalism and expectation for the imminent Second Coming of Christ. Believing that they were living in the end times, they expected God to spiritually renew the Christian Church, and bring to pass the restoration of spiritual gifts and the evangelization of the world. In 1900, Charles Parham, an American evangelist and faith healer, began teaching that speaking in tongues was the Bible evidence of Spirit baptism. Along with William J. Seymour, a Wesleyan-Holiness preacher, he taught that this was the third work of grace. The three-year-long Azusa Street Revival, founded and led by Seymour in Los Angeles, California, resulted in the growth of Pentecostalism throughout the United States and the rest of the world. Visitors carried the Pentecostal experience back to their home churches or felt called to the mission field. While virtually all Pentecostal denominations trace their origins to Azusa Street, the movement has had several divisions and controversies. Early disputes centered on challenges to the doctrine of entire sanctification, as well as that of the Trinity. As a result, the Pentecostal movement is divided between Holiness Pentecostals who affirm the second work of grace, and Finished Work Pentecostals who are partitioned into trinitarian and non-trinitarian branches, the latter giving rise to Oneness Pentecostalism.

Comprising over 700 denominations and many independent churches, Pentecostalism is highly decentralized. No central authority exists, but many denominations are affiliated with the Pentecostal World Fellowship. With over 279 million classical Pentecostals worldwide, the movement is growing in many parts of the world, especially the Global South and Third World countries. Since the 1960s, Pentecostalism has increasingly gained acceptance from other Christian traditions, and Pentecostal beliefs concerning the baptism of the Holy Spirit and spiritual gifts have been embraced by non-Pentecostal Christians in Protestant and Catholic churches through their adherence to the Charismatic movement. Together, worldwide Pentecostal and Charismatic Christianity numbers over 644 million adherents. While the movement originally attracted mostly lower classes in the global South, there is a new appeal to middle classes. Middle-class congregations tend to have fewer members. Pentecostalism is believed to be the fastest-growing religious movement in the world.