The 17-second message tells swimmers they are on CCTV and that jumping from the bridge is “extremely dangerous.”
It will play when CCTV operators spot people who appear to be considering jumping into the river.
The installation of the speaker follows work by the Council, RNLI Teddington, London Fire Brigade and the Met Police.
The footbridge near the busy lock is popular with swimmers but there have been constant warnings about cold water shock and hidden debris below the surface.
Anyone who sees someone in difficulty in the water should call 999.
