Azelaic acid (AzA), or nonanedioic acid, is an organic compound with the formula HOOC(CH₂)₇COOH. This saturated dicarboxylic acid exists as a white powder. It is found in wheat, rye, and barley. It is a precursor to diverse industrial products including polymers and plasticizers, as well as being a component of several hair and skin conditioners. In medicine, topical formulations of AzA are used in the treatment of acne vulgaris, where they exert antimicrobial and keratolytic effects that reduce clogged hair follicles and inflammation. In plants, AzA functions as a mobile signaling molecule that primes systemic acquired resistance against pathogens by inducing the accumulation of salicylic acid. AzA also inhibits tyrosinase.

Azelaic acid is industrially produced by the ozonolysis of oleic acid. The side product is nonanoic acid. It is produced naturally by Malassezia furfur (also known as Pityrosporum ovale), a yeast that lives on normal skin. The bacterial degradation of nonanoic acid gives azelaic acid.

In plants, azelaic acid (AzA) functions as an endogenous signaling molecule that plays a central role in systemic defense responses after infection. It acts as a "distress flare," produced primarily in response to biotic stress such as pathogen attack, and is transported from infected tissues to distant parts of the plant. There, it primes systemic acquired resistance (SAR) against a broad spectrum of pathogens by inducing the accumulation of salicylic acid, a key component of the plant's immune response. Mechanistically, AzA works in concert with other mobile signals, including glycerol-3-phosphate and specific lipid transfer proteins, to orchestrate a systemic defense network that enhances disease resistance and enables rapid defensive responses to subsequent infections.

Azelaic acid treats acne through a multifaceted mechanism that includes antibacterial, anti-keratinizing, and anti-inflammatory activities, as well as direct effects on skin microflora. It exerts bacteriostatic action against Propionibacterium acnes and Staphylococcus epidermidis by disrupting microbial cellular metabolism and membrane pH balance, resulting in reduced bacterial proliferation without promoting resistance. In keratinocytes, azelaic acid inhibits DNA, RNA, and protein synthesis, thus normalizing follicular keratinization, which helps prevent the formation of comedones—clogged hair follicles that can develop into acne lesions and are considered a hallmark of the disease. Additionally, its anti-inflammatory properties decrease the production of reactive oxygen species and pro-inflammatory cytokines, collectively reducing both visible inflammation and the severity of acne lesions.

Esters of this dicarboxylic acid find applications in lubrication and plasticizers. In lubricant industries, it is used as a thickening agent in lithium complex grease. With hexamethylenediamine, azelaic acid forms Nylon-6,9, which finds specialized uses as a plastic.

In 2023, it was the 309th most commonly prescribed medication in the United States, with more than 200,000 prescriptions.

Azelaic acid is used to treat mild to moderate acne, both comedonal acne and inflammatory acne. It belongs to a class of chemicals called dicarboxylic acids. It works by killing acne bacteria that infect skin pores. It also decreases the production of keratin, which is a natural substance that promotes the growth^{[clarification needed]} of acne bacteria. Azelaic acid is also used as a topical gel treatment for rosacea, due to its ability to reduce inflammation. It clears the bumps and swelling caused by rosacea.

In topical pharmaceutical preparations and scientific research AzA is typically used in concentrations between 15% and 20% but some research demonstrates that in certain vehicle formulations, the pharmaceutical effects of 10% Azelaic acid have the potential to be fully comparable to that of some 20% creams.