6-(Diethylamino)-2,3-dihydro-1H-xanthene-4-carbaldehyde (DDXC), a reported synthetic intermediate for near-infrared fluorescent dyes, was developed into a fluorescent pH probe for extreme acidity. The unique sensing mechanism of DDXC for pH is based on the reversible protonation of the carbonyl oxygen followed by keto-enol tautomerization. The probe displays a linear ratiometric fluorescence response (I512/I580) to H+ over the extremely acidic range of pH 2.0-4.0 with a pKa of 3.11, and features high fluorescence quantum yield (Φ = 0.60) and excellent selectivity. More importantly, the probe can be applied to ratiometric fluorescence imaging of pH changes in living cells, making it a potential molecular tool for pH-related cell biology study.