闹钟是一种可设置在指定时间响闹的时钟。闹钟的最基本用途是唤醒睡着的人，但有时也会作为其他日常生活的提醒。如果要令闹钟停止响闹，可按下一个按钮。有些闹钟会在数分钟后自动停止响闹。要在传统的有时针及分针的闹钟设置响闹时间，可转动闹钟后面的钮。

传统的闹钟用力学的原理操作，在闹钟上面设闹铃。近来出现的数字闹钟可发出其他声音。简单的数字闹钟只发出哔哔声；较新颖的能发出更多元化的声音，甚至能说话。有些闹钟可在指定时间开收音机。

An alarm clock (or sometimes just an alarm) is a clock that is designed to make a sound, or some other signal, at a specific time. The primary utility of these clocks is to awaken people from their night's sleep or short naps; they are sometimes used for other reminders as well. Most use sound; some use light or vibration. Some have sensors to identify when a person is in a light stage of sleep, in order to avoid waking someone who is deeply asleep, which causes tiredness, even if the person has had adequate sleep. To stop the sound or light, a button or handle on the clock is pressed; most clocks automatically stop the alarm if left unattended long enough. A classic analog alarm clock has an extra hand or inset dial that is used to specify the time at which to activate the alarm. Alarm clocks are also found on mobile phones and watches.