The acronym SAD may refer to:

• Sagittal Abdominal Diameter, the diameter around one's waistline
• Schizoaffective disorder, a psychiatric diagnosis with symptoms of a mood disorder and schizophrenia
• Seasonal affective disorder, a mood disorder with depressive symptoms in the winter
• Separation anxiety disorder, a psychological condition related to anxiety of separation from home or from people
• Sexual arousal disorder, the inability to attain or maintain typical responses to sexual arousal
• Selected area diffraction, a crystallographic experimental technique that can be performed inside a transmission electron microscope
• Shiromani Akali Dal, an Indian political party
• Silicon Avalanche Diode, an electronic component
• Singles Awareness Day, a humorous holiday celebrated on February 14
• Small angle diffraction, a technique for analysing materials
• Social anxiety disorder, a diagnosis referring to excessive anxiety in social situations
• Software architecture document, a document for the architecture of computer software
• Special Activities Division, the Central Intelligence Agency's special operations force in the United States
• Standard American Diet, a phrase associated with diet-related obesity in the United States
• Sum of absolute differences, a simple metric for motion estimation in video compression
• Systems Analysis and Design, an interdisciplinary part of science

Sad can also mean:

• Ṣād, a letter of the Arabic alphabet (ﺹ).
• Sad (sura), the 38th sura of the Qur'an (named after the letter)
• Sad, Podlaskie Voivodeship (north-east Poland)
• Sad, Dnipropetrovsk Oblast, an urban-type settlement in Dnipropetrovsk Oblast, Ukraine