Christianity is the predominant religion in the United States, although estimates vary among sources. According to a 2024 Gallup survey, approximately 69% of the U.S. population—about 235 million out of 340 million people—identify as Christian. A plurality of Americans identify as Protestant (45%), followed by Roman Catholics (22%). Smaller Christian groups include members of the Church of Jesus Christ of Latter-day Saints (1.5%), Eastern Orthodox Christians (0.5%), and other Christian denominations (0.4%). The United States currently has the largest Christian population in the world and, more specifically, the largest Protestant population globally, comprising nearly 235 million Christians and approximately 153 million adherents of Protestant denominations. However, while the U.S. leads in absolute numbers, several other nations have a greater proportion of their populations identifying as Christian.

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. The 2023-2024 Pew Religious Landscape Survey in the United States found that 40% identitied as Protestant and 19% as Catholic.

All Protestant denominations accounted for 48.5% of the population, making Protestantism the most common form of Christianity in the country and the majority religion in general in the United States, while the Catholic Church by itself, at 22.7% of the population, is the largest individual denomination. The nation's second-largest denomination and the single largest Protestant denomination is the Southern Baptist Convention. Among Eastern Christian denominations, there are several Eastern Orthodox and Oriental Orthodox churches, with just below 1 million adherents in the U.S., or 0.4% of the total population. Christianity is the predominant religion in all U.S. states and territories. Conversion into Christianity has significantly increased among Korean Americans, Chinese Americans, and Japanese Americans in the United States. In 2012, the percentage of Christians in these communities were 71%, 30% and 37% respectively.

Christianity was introduced to the Americas during European settlement beginning in the 16th and 17th centuries. Immigration further increased Christian numbers. Going forward from its foundation, the United States has been called a Protestant nation by a variety of sources. When the categories of "irreligion" and "unaffiliated" are included as religious categories for statistical purposes, Protestantism is technically no longer the religious category of the majority; however, this is primarily the result of an increase in Americans, such as Americans of Protestant descent or background, professing no religious affiliation, rather than being the result of an increase in non-Protestant religious affiliations, and Protestantism remains by far the majority or dominant form of religion in the United States among American Christians and those Americans who declare a religion affiliation. Today, most Christian churches in the United States are either Mainline Protestant, Evangelical Protestant, or Catholic.

Christian denominations in the United States are usually divided into three large groups: two types of Protestantism (Evangelical and Mainline) and Catholicism. There are also Christian denominations, making up a smaller percentage, that do not fall within the confines of these groups, such as Eastern and Oriental Orthodoxy and various restorationist groups such as the Latter Day Saint movement, Adventists and Jehovah's Witnesses.

A 2004 survey of the United States identified the percentages of these groups as 26.3% (Evangelical), 17.5% (Catholics), and 16% (Mainline); the other groups made up 2.7%. In a Statistical Abstract of the United States, based on a 2001 study of the self-described religious identification of the adult population, the percentages for these same groups are 28.6% (Evangelical), 24.5% (Catholics), and 13.9% (Mainline). Christian religious groups made up 76.5% of the total population, while the other religious groups account for 3.7%. According the 2020 ARDA Database, there were approximately 60 million Christians independent from denominations.

In typical usage, the term mainline is contrasted with evangelical.

The Association of Religion Data Archives (ARDA) counts 26,344,933 members of mainline churches versus 39,930,869 members of evangelical Protestant churches. There is evidence that there has been a shift in membership from mainline denominations to evangelical churches. Additionally, ARDA's 2010 study indicated Baptists were the largest Protestant group throughout the United States, followed by non-denominational Protestants. By 2014, the Pew Research Center determined non- and inter-denominational Protestants became the second-largest Christian group with Baptists third. ARDA's 2020 religion census also counted the movement as overtaking Baptists, making up more than 13.1% of the religious population and 6.4% of the general population.