Sidetone is audible feedback to someone who is speaking. The term is most used in telecommunication contexts.

In telephony, sidetone is the effect of sound that is picked up by the telephone's mouthpiece and in real-time introduced at a low level into the earpiece of the same handset, acting as controlled feedback. Sidetone in 19th century telephones varied until the carbon transmitter was used, which produced a distinct sidetone that discouraged speaking loudly enough, and occasionally so loud as to throw the instrument into uncontrolled oscillation or "howling". Sidetone is inactive when phones of any kind are running in speakerphone mode, due to perpetual and almost immediate feedback looping. Anti-sidetone circuitry incorporating the principle of the hybrid coil brought sidetone under control in the early 20th century, leaving enough to assure the user that the phone is really working, and allowing the use of a unitized telephone handset. In cellular technologies, one of the many benefits of sidetone-enabled phones is that a user knows a call has been dropped or ended if he or she no longer hears sidetone.