孤儿药（英语：Orphan drug，亦称为罕用药）指的是一些专门用于治愈（cure）或治疗（控制，treat）罕见疾病的特效药物。取自孤儿孤苦无依且乏人重视的概念。由于孤儿药的市场需求太小，正常情况下药物开发上难以收回成本，导致罕见疾病患者无力负担购药费用。为了鼓励开发商投入资源开发此类药物，各国在社会福利方面多有规定该类药物的开发商可以享有一系列的优惠政策。

例如治疗庞贝氏症的药物Myozyme便是一例，发展出该药的中央研究院生物医学科学研究所所长陈垣崇便是投入研究数十年，也放弃了许多收入更高的工作机会。

An orphan drug is a pharmaceutical agent that has been developed specifically to treat a rare medical condition, the condition itself being referred to as an orphan disease.

In the US and EU it is easier to gain marketing approval for an orphan drug, and there may be other financial incentives, such as extended exclusivity periods, all intended to encourage the development of drugs which might otherwise lack a sufficient profit motive. The assignment of orphan status to a disease and to any drugs developed to treat it is a matter of public policy in many countries, and has resulted in medical breakthroughs that may not have otherwise been achieved due to the economics of drug research and development.