Protestantism is the largest grouping of Christians in the United States, with its combined denominations collectively comprising about 43% of the country's population in 2019. Other estimates suggest that 48.5% of the U.S. population is Protestant. Simultaneously, this corresponds to around 20% of the world's total Protestant population. The U.S. contains the largest Protestant population of any country in the world. Baptists comprise about one-third of American Protestants. The Southern Baptist Convention is the largest single Protestant denomination in the U.S., comprising one-tenth of American Protestants. Twelve of the original Thirteen Colonies were Protestant, with only Maryland having a sizable Catholic population due to Lord Baltimore's religious tolerance.

Christianity is the most prevalent religion in the United States. Estimates from 2021 suggest that of the entire U.S. population about 63% is Christian. The majority of Christian Americans are Protestant Christians, though there are also significant numbers of American Roman Catholics and other Christian denominations such as The Church of Jesus Christ of Latter-day Saints, Eastern Orthodox Christians and Oriental Orthodox Christians, and Jehovah's Witnesses. The United States has the largest Christian population in the world and, more specifically, the largest Protestant population in the world, with nearly 210 million Christians and, as of 2021, over 140 million people affiliated with Protestant churches, although other countries have higher percentages of Christians among their populations. The Public Religion Research Institute's "2020 Census of American Religion", carried out between 2014 and 2020, showed that 70% of Americans identified as Christian during this seven-year interval. In a 2020 survey by the Pew Research Center, 65% of adults in the United States identified themselves as Christians. They were 75% in 2015, 70.6% in 2014, 78% in 2012, 81.6% in 2001, and 85% in 1990. About 62% of those polled claim to be members of a church congregation.