A Sunday school, known as a Sabbath school in some sabbatarian Christian denominations, is an educational institution or weekly activity in a place of worship, usually Christian in character and often intended for children or neophytes.

Sunday school classes usually precede a Sunday church service and are used to provide catechesis to Christians, especially children and teenagers, and sometimes adults as well. Churches of many Christian denominations have classrooms attached to the church used for this purpose. Many Sunday school classes operate on a set curriculum, with some teaching attendees a catechism. Members often receive certificates and awards for participation, as well as attendance.

Sunday school classes may provide a light breakfast. On days when Holy Communion is being celebrated, however, some Christian denominations encourage fasting before receiving the Eucharistic elements.

Sunday schools in Europe began with the Catholic Church's Confraternity of Christian Doctrine, founded in the 16th century by the archbishop Charles Borromeo to teach young Italian children the faith.

Protestant Sunday schools were first set up in the 18th century in England to provide education to working children. William King started a Sunday school in 1751 in Dursley, Gloucestershire. Robert Raikes, editor of the Gloucester Journal, started a similar one in Gloucester in 1781. He wrote an article in his journal, and as a result many clergymen supported schools, which aimed to teach the youngsters reading, writing, cyphering (doing arithmetic) and a knowledge of the Bible.

The Sunday School Society was founded by Baptist deacon William Fox on 7 September 1785 in Prescott Street Baptist Church of London. The latter had been touched by articles of Raikes, on the problems of youth crime. Pastor Thomas Stock and Raikes have thus registered a hundred children from six to fourteen years old. The society has published its textbooks and brought together nearly 4,000 Sunday schools.

In 1785, 250,000 English children were attending Sunday school. There were 5,000 in Manchester alone. By 1835, the Sunday School Society had distributed 91,915 spelling books, 24,232 New Testaments and 5,360 Bibles. The Sunday school movement was cross-denominational. Financed through subscription, large buildings were constructed that could host public lectures as well as provide classrooms. Adults would attend the same classes as the infants, as each was instructed in basic reading. In some towns, the Methodists withdrew from the large Sunday school and built their own. The Anglicans set up their National schools that would act as Sunday schools and day schools. These schools were the precursors to a national system of education.

The educational role of the Sunday schools ended with the Education Act 1870, which provided universal elementary education. In the 1920s they also promoted sports, and ran Sunday school leagues. They became social centres hosting amateur dramatics and concert parties. By the 1960s, the term Sunday school could refer to the building and rarely to the activities inside. By the 1970s even the largest Sunday school had been demolished. The locution today chiefly refers to catechism classes for children and adults that occur before the start of a church service. In certain Christian traditions, in certain grades, for example the second grade or eighth grade, Sunday school classes may prepare youth to undergo a rite such as First Communion or Confirmation. The doctrine of Sunday Sabbatarianism, held by many Christian denominations, encourages practices such as Sunday school attendance, as it teaches that the entirety of the Lord's Day should be devoted to God; as such many children and teenagers often return to the church in the late afternoon for youth group before attending an evening service of worship.