在音乐记谱中，四分音符是一种音符时值。在五线谱记法中，四分音符表示为一个实心的椭圆符头加上一个不带符尾的符干（如右图左边的记号）。当符头位于五线谱第三线上方（包括第三线）时，符干朝下，否则符干朝上，符干改变方向时，符头的方向也要作相应改变（见图）。四分音符的音长是全音符的四分之一，即4/4拍中的一拍。二分音符的音长是其2倍，八分音符的音长是其二分之一，十六分音符的音长是其四分之一，等等。

在Unicode中，四分音符的编码是U+2669（♩）。

在休止符中，对应有着同样音长的是四分休止符，在五线谱中表示为一个类似将“z”和“c”从上至下连起来的记号，写在线谱中央（如右图右边的记号）。

A quarter note (American) or crotchet (British, from the sense 'hook') is a note played for one quarter of the duration of a whole note (or semibreve). Often, musicians will say that a crotchet is one beat, but this is not always correct, as the beat is indicated by the time signature of the music; a quarter note may or may not be the beat. Quarter notes are notated with a filled-in oval note head and a straight, flagless stem. The stem usually points upwards if it is below the middle line of the stave or downwards if it is on or above the middle line. However, the stem direction may differentiate more than one part. The head of the note also reverses its orientation in relation to the stem. (See image.)