We developed a rhodamine–rhodanine-based pH probe with pK_a = 4.85 in buffer solution. The fluorescence intensity exhibited strong pH-dependent performance and responded linearly to minor pH fluctuations within the range of 4.2–5.2. In addition, the fluorescence microscopic images suggested this probe had excellent cell membrane permeability and could image weak acid pH changes of lysosomes in live cells without auto-fluorescence and interference from the complex intracellular environment. The results demonstrated that the probe had great potential in monitoring H⁺ in vitro and in living cells.