Dysfunction of the human ether-a-go-go related gene (hERG)-encoded potassium channel is identified as a major cause of the long QT syndrome, a marker for lethal cardiac arrhythmia. Furthermore, recent studies revealed that hERG K+ channel is a regulator of tumor cell apoptosis and proliferation. Herein, an ultrasensitive fluorescence assay combining DNA-functionalized gold nanoparticles and rolling circle amplification (RCA) was attempted for the first time to visualize hERG channels in living cells. The spherical nucleic acid gold nanoparticles, which can anchor on hERG channels in the cell membrane, not only act as the primary amplification elements but also trigger the subsequent RCA reaction to achieve the secondary amplification. Within 30 min, the ratio of reporter to target can reach up to 104, realizing the detection of hERG channels in cells with low-level expression. Therefore, the strategy provides a valuable tool for hERG-related studies. More importantly, it opens a new horizon for imaging various membrane proteins which possess specific aptamers or antibodies.