Fatty acid desaturases (also called unsaturases) are a family of enzymes that convert saturated fatty acids into unsaturated fatty acids and polyunsaturated fatty acids. For the common fatty acids of the C18 variety, desaturases convert stearic acid into oleic acid. Other desaturases convert oleic acid into linoleic acid, which is the precursor to alpha-linolenic acid, gamma-linolenic acid, and eicosatrienoic acid.

Two subgroups of desaturases are recognized:

For example, Δ6 desaturation introduces a double bond between carbons 6 and 7 of linoleic acid (LA C₁₈H₃₂O₂; 18:2-n6) and α-linolenic acid (ALA: C₁₈H₃₀O₂; 18:3-n3), creating γ-linolenic acid (GLA: C₁₈H₃₀O₂,18:3-n6) and stearidonic acid (SDA: C₁₈H₂₈O₂; 18:4-n3) respectively.

In the biosynthesis of essential fatty acids, an elongase alternates with various desaturases (for example, Δ6-desaturase) repeatedly inserts an ethyl group, then forms a double bond.

Desaturases have diiron active sites reminiscent of methane monooxygenase. These enzymes are O₂-dependent, consistent with their function as either hydroxylation or oxidative dehydrogenation.

Desaturases produce unsaturated fatty acids. Unsaturated fatty acids help maintain structure and function of membranes. Highly unsaturated fatty acids (HUFAs) are incorporated into phospholipids and participate in cell signaling.

Unsaturated fatty acids and their derived fats increase the fluidity of membranes.

Fatty acid desaturase appear in all organisms: for example, bacteria, fungus, plants, animals and humans. Four desaturases occur in humans: Δ9-desaturase, Δ6-desaturase, Δ5-desaturase, and Δ4-desaturase.