主题曲或主题歌，是在电影、电视剧、动画、话剧等作品中，用来代表作品的主要乐曲或歌曲，不过不一定会在这些作品中播放，如某些电影放置在电影结束后的字幕清单中（如电影《阿凡达》的主题曲《我看见你》即是一个显著的例子）。有歌词可以演唱的叫做主题歌；无歌词只有曲子，不能演唱的叫做主题曲。

在不同国家上演、播放的作品，有时会由于不同的风俗民情等因素，会另请填词人以本土语言重新填写歌词，并会另请歌手、配音员或其他人士代唱。例如欧美地区，几乎都会把歌曲与歌词重新改写，影像则是会和原作品有部分差异。台湾、香港动画歌曲中，亦有部分歌曲有改唱（使用原曲，以本地语言填词和主唱）。

Theme music is a piece that is often written specifically for a radio program, television program, video game or movie, and usually played during the intro, title sequence and/or ending credits.

The phrase theme song or signature tune may also be used to refer to a signature song that has become especially associated with a particular performer or dignitary; often used as they make an entrance. The purpose of a theme song is often similar to that of a leitmotif. Such songs can also be used in other ways. One author has made extensive use of them in an effort to explore the feelings behind world views.