Hydrogen sulfide (H2S) is an important biological messenger, which recently joined nitric oxide (NO) and carbon monoxide (CO) as an endogenously produced gasotransmitter. This discovery resulted in a renewed interest towards H2S and, in particular, in its reactivity with biological and bioinorganic targets. However, in the scientific literature studies exploring the reactivity of H2S/HS- with non-heme proteins are rare. In this work, we studied the interaction of HS- with the natural metalloprotein Cu-azurin and its cobalt variant via UV-Vis, fluorescence, and EPR spectroscopy, as well as X-ray diffraction. Interaction of HS- significantly enhances the fluorescence emission of fluorescently labelled copper azurin in buffered solutions and at physiological pH. In contrast, fluorescently labelled cobalt azurin undergoes a fluorescence quenching in the presence of HS- in line with the changes observed in the absorption spectrum of the construct. The results highlight the potential of azurin for implementation in a fast and simple HS- sensor.