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 3′-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.