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miRNA (miR)‐155 is a potential biomarker for breast cancers. We aimed at developing a nanosensor for miR‐155 detection by integrating hybridization chain reaction (HCR) and silver nanoclusters (AgNCs). HCR serves as an enzyme‐free and isothermal amplification method, whereas AgNCs provide a built‐in fluorogenic detection probe that could simplify the downstream analysis. The two components were integrated by adding a nucleation sequence of AgNCs to the hairpin of HCR. The working principle was based on the influence of microenvironment towards the hosted AgNCs, whereby unfolding of hairpin upon HCR has manipulated the distance between the hosted AgNCs and cytosine‐rich toehold region of hairpin. As such, the dominant emission of AgNCs changed from red to yellow in the absence and presence of miR‐155, enabling a ratiometric measurement of miR with high sensitivity. The limit of detection (LOD) of our HCR‐AgNCs nanosensor is 1.13 fM in buffered solution. We have also tested the assay in diluted serum samples, with comparable LOD of 1.58 fM obtained. This shows the great promise of our HCR‐AgNCs nanosensor for clinical application. [ABSTRACT FROM AUTHOR]