被害妄想症是精神病的其中一种病征，常见于精神分裂症及妄想症的患者，是妄想中最常见的类型。患者会自觉受到迫害，如言语上的针对、嘲弄、跟踪及监听等，患者更可发展出很广泛及系统性的被迫害思想，例如会认为整个暗杀集团部署暗杀行动。故此患者会处处防备，生活极度谨慎，由于担心被害，患者的社交生活及日常工作都会受到影响。

“妄想”是指病人整天多疑多虑，胡乱推理和判断，思维发生障碍，是精神疾病的一个重要症状。患者可伴有幻觉，但无其他明显的精神症状。病人由于缺乏安全感，导致对外界的极度不信任，而产生的一种幻想。多发于老年人或者某些曾经受过超出自我承受能力范围的打击的人。

Persecutory delusions are a set of delusional conditions in which the affected person believes they are being persecuted. Specifically, they have been defined as containing two central elements:

According to the DSM-IV-TR, persecutory delusions are the most common form of delusions in paranoid schizophrenia, where the person believes "he or she is being tormented, followed, tricked, spied on, or ridiculed." They are also often seen in schizoaffective disorder and, as recognized by DSM-IV-TR, constitute the cardinal feature of the persecutory subtype of delusional disorder, by far the most common. Delusions of persecution may also appear in manic and mixed episodes of bipolar disorder and in severe depressive episodes with psychotic features, particularly when associated with bipolar disorder.