The Holiness code is used in biblical criticism to refer to Leviticus chapters 17–26, and sometimes passages in other books of the Pentateuch, especially Numbers and Exodus. It is so called due to its highly repeated use of the word holy (Hebrew: קדוש qəḏōš or kadash). Kadash is usually translated as "holy", but originally meant "set apart", with "special", "clean/pure", "whole" and "perfect" as associated meanings. The term Holiness Code was first coined as the Heiligkeitsgesetz (literally "Holiness Law"; the word 'code' therefore means criminal code) by German theologian August Klostermann in 1877. Critical biblical scholars have regarded it as a distinct unit and have noted that the style is noticeably different from the main body of Leviticus. Unlike the remainder of Leviticus, the many laws of the Holiness Code are expressed very closely packed together, and very briefly.

According to most versions of the documentary hypothesis, the Holiness Code represents an earlier text that was edited and incorporated into the Priestly source and the Torah as a whole, although some scholars, such as Israel Knohl, believe the Holiness Code to be a later addition to the Priestly source. This source is often abbreviated as "H". A generally accepted date is sometime in the seventh century BC, when it presumably originated among the priests in the Temple in Jerusalem.

The Holiness Code also uses a noticeably different choice of vocabulary, repeating phrases such as I, Yahweh, am holy; I am Yahweh; and I am Yahweh, who makes you holy, an unusually large number of times. Additionally, Leviticus 17 begins with This is the thing which Yahweh has commanded, saying.., and Leviticus 26 strongly resembles the conclusion of a law code, despite the presence of further laws afterward, such as at Leviticus 27, giving the Holiness Code the appearance of a single distinct unit.

Professor Christine Hayes discusses a difference between the Holiness Code and the rest of Leviticus: in the Holiness Code, Israel itself is regarded as holy, not just the priestly class:

This theme, and the exhortation, “you shall be holy, for I the Lord your God am holy,” they find their fullest expression in the block of text; Leviticus 17 through 26 that’s referred to as the Holiness Code. There’s an important difference between Leviticus 1 through 16 and the Holiness Code. According to Leviticus 1 through 16, Israel’s priests are designated as holy: a holy class within Israel, singled out, dedicated to the service of God and demarcated by rules that apply only to them. Israelites may aspire to holiness, but it’s not assumed. However, in the Holiness Code, we have texts that come closer to the idea that Israel itself is holy by virtue of the fact that God has set Israel apart from the nations to himself, to belong to him, just as he set apart the seventh day to himself to belong with him.