Olanzapine (brand name Zyprexa) is an atypical antipsychotic drug belonging to the thienobenzodiazepine family, made by Eli Lilly & Co. It is a yellow crystaline solid, with chemical formula C17H20N4S. It is not yet known, but it most likely works as a dopamine antagonist as well as a serotonin 5-HT2 antagonist.

Olanzapine was the first of the atypical antipsychotics to be approved for long-term treatment of schizophrenia. It has been prescribed to over 6 million patients in 84 countries since it became available in 1996. Olanzapine is also approved for the short-term treatment of bipolar mania. The most common side effects are somnolence and weight gain.