A sermon is a religious discourse or oration by a preacher, usually a member of clergy. Sermons address a scriptural, theological, or moral topic, usually expounding on a type of belief, law, or behavior within both past and present contexts. Elements of the sermon often include exposition, exhortation, and practical application. The act of delivering a sermon is called preaching. In secular usage, the word sermon may refer, often disparagingly, to a lecture on morals.

In Christian practice, a sermon is usually preached to a congregation in a place of worship, either from an elevated architectural feature, known as a pulpit or an ambo, or from behind a lectern. The word sermon comes from a Middle English word which was derived from Old French, which in turn originates from the Latin word sermō meaning 'discourse.' A sermonette is a short sermon (usually associated with television broadcasting, as stations would present a sermonette before signing off for the night). The Christian Bible contains many speeches without interlocution, which some take to be sermons: Jesus' sermon on the mount in Matthew 5–7 (though the gospel writers do not specifically call it a sermon; the popular descriptor for Jesus' speech there came much later); and Peter after Pentecost in Acts 2:14–40 (though this speech was delivered to non-Christians and as such is not quite parallel to the popular definition of a sermon).

In Islam, sermons are known as khutbah.