航空器注册编号（Aircraft registration），或称「机身编号」、「机尾编号」（tail number），指的是民用航空器在使用前向一国的民航管理机构注册所获发的编号，就如同汽车有车牌号码一样。1944年的《芝加哥民航条约》（Chicago Convention）规定了各国的航空器要有注册编号才可使用。而由于航空器常会跨国飞行，为了便于分辨，每个国家都有其首码（prefix）。

注册编号通常会漆在机身侧边的前后，而有些国家还会规定固定翼飞机要将注册编号漆在主翼的上方或下方，另外在鼻轮盖及垂直尾翼也会漆上注册编号的一部份。大部份国家的首码与编号会以连字号隔开，但美国、日本、韩国等国则会相连在一起不以连字号隔开。

In accordance with the Convention on International Civil Aviation (also known as the Chicago Convention), all civil aircraft must be registered with a national aviation authority (NAA) using procedures set by each country. Every country, even those not party to the Chicago Convention, has a NAA whose functions include the registration of civil aircraft. An aircraft can only be registered once, in the one jurisdiction. The NAA allocates a unique alphanumeric string to identify the aircraft, which also indicates the nationality (ie., country of registration) of the aircraft, and provides a legal document called a Certificate of Registration, one of the documents which must be carried when the aircraft is in operation.