Furan is a heterocyclic organic compound, consisting of a five-membered aromatic ring with four carbon atoms and one oxygen atom. Chemical compounds containing such rings are also referred to as furans.

Furan is a colorless, flammable, highly volatile liquid with a boiling point close to room temperature. It is soluble in common organic solvents, including alcohol, ether, and acetone, and is slightly soluble in water. Its odor is "strong, ethereal; chloroform-like". It is toxic and may be carcinogenic in humans. Furan is used as a starting point for other speciality chemicals.

The name "furan" comes from the Latin furfur, which means bran (furfural is produced from bran). The first furan derivative to be described was 2-furoic acid, by Carl Wilhelm Scheele in 1780. Another important derivative, furfural, was reported by Johann Wolfgang Döbereiner in 1831 and characterised nine years later by John Stenhouse. Furan itself was first prepared by Heinrich Limpricht in 1870, although he called it "tetraphenol" (as if it were a four-carbon analog to phenol, C₆H₅OH).

Industrially, furan is manufactured by the palladium-catalyzed decarbonylation of furfural, or by the copper-catalyzed oxidation of 1,3-butadiene:

In the laboratory, furan can be obtained from furfural by oxidation to 2-furoic acid, followed by decarboxylation. It can also be prepared directly by thermal decomposition of pentose-containing materials, and cellulosic solids, especially pine wood.

The Feist–Benary synthesis is a classic way to synthesize furans. The reaction involves alkylation of 1,3-diketones with α-bromoketones followed by dehydration of an intermediate hydroxydihydrofuran. The other traditional route involve the reaction of 1,4-diketones with phosphorus pentoxide (P₂O₅) in the Paal–Knorr synthesis.

Many routes exist for the synthesis of substituted furans.

Furan has aromatic character because one of the lone pairs of electrons on the oxygen atom is delocalized into the ring, creating a 4n + 2 aromatic system (see Hückel's rule). The aromaticity is modest relative to that for benzene and related heterocycles thiophene and pyrrole. The resonance energies of benzene, pyrrole, thiophene, and furan are, respectively, 152, 88, 121, and 67 kJ/mol (36, 21, 29, and 16 kcal/mol). Thus, these heterocycles, especially furan, are far less aromatic than benzene, as is manifested in the lability of these rings. The molecule is flat but the C=C groups attached to oxygen retain significant double bond character. The other lone pair of electrons of the oxygen atom extends in the plane of the flat ring system.