This is the first report of the fact that absorption of a cw laser beam by a high‐pressure arc may result in a fluorescence signal from the arc that has a negative sign. The laser‐induced fluorescence (LIF) is detected at a number of optical sodium and mercury lines, while the wavelength of the laser is tuned in the wings of the Na‐Dlines. The negative fluorescence signal is found if the laser beam is chopped at frequencies below about 100 Hz. The amplitude and the sign of the fluorescence signal appear to depend strongly on the relative spatial positions of the laser beam and the arc volume from which the fluorescence is detected. At higher chopper frequencies the generally expected positive LIF is regained. The frequency region where the transition occurs depends on the total gas pressure in the arc. A model is presented which explains the negative fluorescence signal. In this model the absorbed laser power affects the electrical conductivity of the plasma. This causes a distortion of the electrical current distribution, leading to a change in the electrical heating of the gas in the arc. Based on this model, results are calculated which qualitatively agree with the experimental results. We discuss the consequences of the discovery of the negative fluorescence for the application of LIF in arc diagnostics.