Social anxiety disorder (SAD), also known as social phobia, is an anxiety disorder characterized by sentiments of fear and anxiety in social situations, causing considerable distress and impairing ability to function in at least some aspects of daily life. These fears can be triggered by perceived or actual scrutiny from others. Individuals with social anxiety disorder fear negative evaluations from other people.

Physical symptoms often include excessive blushing, excessive sweating, trembling, palpitations, rapid heartbeat, muscle tension, shortness of breath, and nausea. Panic attacks can also occur under intense fear and discomfort. Some affected individuals may use alcohol or other drugs to reduce fears and inhibitions at social events. It is common for those with social phobia to self-medicate in this fashion, especially if they are undiagnosed, untreated, or both; this can lead to alcohol use disorder, eating disorders, or other kinds of substance use disorders. According to ICD-10 guidelines, the main diagnostic criteria of social phobia are fear of being the focus of attention, or fear of behaving in a way that will be embarrassing or humiliating, avoidance and anxiety symptoms. Standardized rating scales can be used to screen for social anxiety disorder and measure the severity of anxiety.

The first line of treatment for social anxiety disorder is cognitive behavioral therapy (CBT). CBT is effective in treating this disorder, whether delivered individually or in a group setting. The cognitive and behavioral components seek to change thought patterns and physical reactions to anxiety-inducing situations.

The attention given to social anxiety disorder has significantly increased since 1999 with the approval and marketing of drugs for its treatment. Prescribed medications include several classes of antidepressants: selective serotonin reuptake inhibitors (SSRIs), serotonin–norepinephrine reuptake inhibitors (SNRIs), and monoamine oxidase inhibitors (MAOIs). Other commonly used medications include beta blockers and benzodiazepines. Medications such as SSRIs are effective for social phobia, such as paroxetine.

Literary descriptions of shyness can be traced back to the days of Hippocrates around 400 BC. Hippocrates described someone who "through bashfulness, suspicion, and timorousness, will not be seen abroad; loves darkness as life and cannot endure the light or to sit in lightsome places; his hat still in his eyes, he will neither see, nor be seen by his good will. He dare not come in company for fear he should be misused, disgraced, overshoot himself in gesture or speeches, or be sick; he thinks every man observes him."

The first mention of the psychiatric term "social phobia" (phobie des situations sociales) was made in the early 1900s. Psychologists used the term "social neurosis" to describe extremely shy patients in the 1930s. After extensive work by Joseph Wolpe on systematic desensitization, research on phobias and their treatment grew. The idea that social phobia was a separate entity from other phobias came from the British psychiatrist Isaac Marks in the 1960s. This was accepted by the American Psychiatric Association and was first officially included in the third edition of the Diagnostic and Statistical Manual of Mental Disorders.

After a call to action by psychiatrist Michael Liebowitz and clinical psychologist Richard Heimberg, there was an increase in attention to and research on the disorder. The DSM-IV gave social phobia the alternative name "social anxiety disorder". Research on the psychology and sociology of everyday social anxiety continued. Cognitive behavioural models and therapies were developed for social anxiety disorder. In the 1990s, paroxetine became the first prescription drug in the US approved to treat social anxiety disorder, with others following.

The 10th version of the International Classification of Diseases (ICD-10) classifies social anxiety as a mental and behavioral disorder.