Almorexant (INN, codenamed ACT-078573) is an orexin antagonist, functioning as a competitive receptor antagonist of the OX₁ and OX₂orexin receptors, which was being developed by the pharmaceutical companies Actelion and GSK for the treatment of insomnia. Development of the drug was abandoned in January 2011.

Originally developed by Actelion, from 2007 almorexant was being reported as a potential blockbuster drug, as its novel mechanism of action (orexin receptor antagonism) was thought to produce better quality sleep and fewer side effects than the traditional benzodiazepine and z drugs which dominated the multi-billion dollar insomnia medication market.

In 2008, pharmaceutical giant GlaxoSmithKline bought the development and marketing rights for almorexant from Actelion for an initial payment of $147 million. The deal was worth a potential $3.2billion if the drug were to successfully complete clinical development and obtain FDA approval. GSK and Actelion continued to develop the drug together, and completed a Phase III clinical trial in November 2009.