Ethidium bromide (EtBr) is a dye commonly used to label nucleic acids in molecular biology laboratories. Ethidium bromide belongs to the phenanthridinium family and has the molecular formula C₂₁H₂₀B r N₃. 

 When exposed to ultraviolet radiation, ethidium bromide fluoresces with a red-orange colour, 20 times more intense when bound to DNA. This effect is due to an increase in the hydrophobicity of the environment, rather than to a rigidification of the benzene ring.

Ethidium bromide is mainly used as a DNA intercalating agent, i.e. it is inserted between the base pairs of double-stranded DNA, enabling it to mark the DNA when it is exposed to ultraviolet radiation. It is, however, considered to be mutagenic and carcinogenic, but no study has demonstrated its carcinogenic effect. It is one of the so-called "CMR" substances (Carcinogenic, Mutagenic for germ cells, or toxic for reproduction).

Due to its lightness, safer alternatives, such as GelRed, have been developed. GelRed is a fluorescent nucleic acid dye designed to replace ethidium bromide in the staining of double-stranded DNA, single-stranded DNA or RNA in agarose gels.