We herein describe a rapid and label-free strategy for the sensitive detection of Hg2+ based on target-triggered exponential strand displacement amplification (eSDA). The developed strategy utilizes specially designed DNA probes that form mismatched thymine-thymine (T-T) base pairs at their 30-ends. In the absence of Hg2+, the mismatched T-T base pairs prohibit the DNA polymerase-mediated extension and the subsequent eSDA does not occur. On the other hand, the presence of Hg2+ that mediates the formation of stable T-Hg2+-T base pairs, enables the extension of DNA probes by DNA polymerase. As a result, a large number of duplexes are produced through the eSDA, which is simply monitored in real-time by the gradual fluorescence enhancement from SYBR green I. By employing this strategy, we detected Hg2+ down to 2.95 pM within 30 min, and the practical applicability of this method was successfully demonstrated by detecting Hg2+ in tap water.