Asperger syndrome (AS), also known as Asperger's, is a neurodevelopmental condition characterized by significant difficulties in social interaction and nonverbal communication, along with restricted and repetitive patterns of behaviour and interests. The syndrome is no longer recognised as a diagnosis in itself, having been merged with other conditions into autism spectrum disorder (ASD). It was considered to differ from other diagnoses that were merged into ASD by relatively unimpaired spoken language and intelligence.

The syndrome was named after the Austrian pediatrician Hans Asperger, who, in 1944, described children in his care who struggled to form friendships, did not understand others' gestures or feelings, engaged in one-sided conversations about their favourite interests, and were clumsy. In 1994, the diagnosis of Asperger's was included in the fourth edition (DSM-IV) of the American Diagnostic and Statistical Manual of Mental Disorders; however, with the publication of DSM-5 in 2013 the syndrome was removed, and the symptoms are now included within autism spectrum disorder along with classic autism and pervasive developmental disorder not otherwise specified (PDD-NOS). It was similarly merged into autism spectrum disorder in the International Classification of Diseases (ICD-11) as of 2021.

The exact cause of Asperger's is poorly understood. While it has high heritability, the underlying genetics have not been determined conclusively. Environmental factors are also believed to play a role. Brain imaging has not identified a common underlying condition. There is no single treatment, and the UK's National Health Service (NHS) guidelines suggest that 'treatment' of any form of autism should not be a goal, since autism is not 'a disease that can be removed or cured'. According to the Royal College of Psychiatrists, while co-occurring conditions might require treatment, 'management of autism itself is chiefly about the provision of the education, training and social support/care required to improve the person's ability to function in the everyday world'. The effectiveness of particular interventions for autism is supported by only limited data. Interventions may include social skills training, cognitive behavioral therapy, physical therapy, speech therapy, parent training, and medications for associated problems, such as mood or anxiety. Autistic characteristics tend to become less obvious in adulthood, but social and communication difficulties usually persist.

In 2015, Asperger's was estimated to affect 37.2 million people globally, or about 0.5% of the population. The exact percentage of people affected has still not been firmly established. Autism spectrum disorder is diagnosed in males more often than females, and females are typically diagnosed at a later age. The modern conception of Asperger syndrome came into existence in 1981, and went through a period of popularization. It became a standardized diagnosis in the 1990s, and was retired as a diagnosis in 2013. Many questions and controversies about the condition remain.