臂板信号机是最早出现的铁路信号机。也是铁路固定信号机的一种。白天用臂板的不同位置（水平还是斜向），夜间或视线不清时用不同颜色的灯光显示信号，适合于没有可靠交流电源的车站使用，只需要电池即可。臂板信号机的灯光很暗，一般规定夜间行车时臂板信号机的灯光信号要机车乘务组二人以上确认。在中国，臂板信号机还可以在通车很少的专用线上看到。

每个臂板单元包含两个部分：一条可旋转至固定位置的悬臂；一组包含灯光光源与位于灯光前部可变换位置的灯光信号表示器用彩色玻璃。

按单个信号机立柱上安装臂板的数目，可分为单臂板信号机、双臂板信号机和三臂板信号机。

One of the earliest forms of fixed railway signal is the semaphore. These signals display their different indications to train drivers by changing the angle of inclination of a pivoted 'arm'. Semaphore signals were patented in the early 1840s by Joseph James Stevens, and soon became the most widely used form of mechanical signal. Designs have altered over the intervening years, and colour light signals have replaced semaphore signals in some countries, but in others they remain in use. Note that this article doesn't represent a world wide view of the semaphore.