Molecular imaging is an essential tool for disease diagnostics and treatment. Direct imaging of low-abundance nucleic acids in living cells remains challenging because of the relatively low sensitivity and insufficient signal-to-background ratio of conventional molecular imaging probes. Herein, we report a class of DNA-templated gold nanoparticle (GNP)-quantum dot (QD) assembly-based probes for catalytic imaging of cancer-related microRNAs (miRNA) in living cells with signal amplification capacity. We show that a single miRNA molecule could catalyze the disassembly of multiple QDs with the GNP through a DNA-programmed thermodynamically driven entropy gain process, yielding significantly amplified QD photoluminescence (PL) for miRNA imaging. By combining the robust PL of QDs with the catalytic amplification strategy, three orders of magnitude improvement in detection sensitivity is achieved in comparison with non-catalytic imaging probe, which enables facile and accurate differentiation between cancer cells and normal cells by miRNA imaging in living cells.