Hypochondriasis or hypochondria is a condition in which a person is excessively and unduly worried about having a serious illness. Hypochondria is an old concept whose meaning has repeatedly changed over its lifespan. It has been claimed that this debilitating condition results from an inaccurate perception of the condition of body or mind despite the absence of an actual medical diagnosis. An individual with hypochondriasis is known as a hypochondriac. Hypochondriacs become unduly alarmed about any physical or psychological symptoms they detect, no matter how minor the symptom may be, and are convinced that they have, or are about to be diagnosed with, a serious illness.

Often, hypochondria persists even after a physician has evaluated a person and reassured them that their concerns about symptoms do not have an underlying medical basis or, if there is a medical illness, their concerns are far in excess of what is appropriate for the level of disease. It is also referred to hypochondriaism which is the act of being in a hypochondriatic state, acute hypochondriaism. Many hypochondriacs focus on a particular symptom as the catalyst of their worrying, such as gastro-intestinal problems, palpitations, or muscle fatigue. To qualify for the diagnosis of hypochondria the symptoms must have been experienced for at least six months.

International Classification of Diseases (ICD-10) classifies hypochondriasis as a mental and behavioral disorder. In the Diagnostic and Statistical Manual of Mental Disorders, DSM-IV-TR defined the disorder, "Hypochondriasis", as a somatoform disorder and one study has shown it to affect about 3% of the visitors to primary care settings. The 2013 DSM-5 replaced the diagnosis of hypochondriasis with the diagnoses of somatic symptom disorder (75%) and illness anxiety disorder (25%).

Hypochondria is often characterized by fears that minor bodily or mental symptoms may indicate a serious illness, constant self-examination and self-diagnosis, and a preoccupation with one's body. Many individuals with hypochondriasis express doubt and disbelief in the doctors' diagnosis, and report that doctors’ reassurance about an absence of a serious medical condition is unconvincing, or short-lasting. Additionally, many hypochondriacs experience elevated blood pressure, stress, and anxiety in the presence of doctors or while occupying a medical facility, a condition known as "white coat syndrome". Many hypochondriacs require constant reassurance, either from doctors, family, or friends, and the disorder can become a debilitating challenge for the individual with hypochondriasis, as well as their family and friends. Some individuals with hypochondria completely avoid any reminder of illness, whereas others frequently visit medical facilities, sometimes obsessively. Some may never speak about it.