DNAzymes are a promising platform for metal ion detection, and a few DNAzyme‐based sensors have been reported to detect metal ions inside cells. However, these methods required an influx of metal ions to increase their concentrations for detection. To address this major issue, the design of a catalytic hairpin assembly (CHA) reaction to amplify the signal from photocaged Na+‐specific DNAzyme to detect endogenous Na+ inside cells is reported. Upon light activation and in the presence of Na+, the NaA43 DNAzyme cleaves its substrate strand and releases a product strand, which becomes an initiator that trigger the subsequent CHA amplification reaction. This strategy allows detection of endogenous Na+ inside cells, which has been demonstrated by both fluorescent imaging of individual cells and flow cytometry of the whole cell population. This method can be generally applied to detect other endogenous metal ions and thus contribute to deeper understanding of the role of metal ions in biological systems. Cha cha cha: A catalytic hairpin assembly (CHA) reaction was designed to amplify the signal from photocaged Na+‐specific DNAzyme cleavage to detect endogenous Na+ inside cells. After light activation and in the presence of Na+, the NaA43 DNAzyme cleaves its substrate strand and releases a product strand, which becomes an initiator that can trigger the subsequent CHA signal amplification reaction.