Mercury contamination is one of the most severe issues in society due to its threats to public health and the ecological system. However, traditional methods for mercury ion detection are still limited by their time-consuming procedures, requirement of expensive instruments, and low selectivity. In recent decades, tremendous progress has been made in the development of functional nucleic acid-based, especially DNAzyme sensors for mercury (Ⅱ) (Hg) determination, including RNA-cleaving DNAzymes and G-quadruplex-based DNAzymes in particular. Researchers have heavily studied the construction of Hg sensors, mainly originating from in vitro selection-derived DNAzymes, by incorporating T-Hg-T recognition moieties in existing DNAzyme scaffolds, and interfacing Hg-sensitive sequences with nanomaterials. In the last case, the employment of materials (as quenchers, signal transducers and DNA immobilizers) enriches the application scenarios of current Hg-DNAzymes, due to a combination of their functions. We summarize a broad range of sensing approaches, including optical, electrochemical, and other sensing methods, and compare their features. This review elaborates on the rational design strategies for engineering DNAzymes to selectively sense Hg, critically discusses their properties in different application scenarios, and summarizes recent advances in this field. Additionally, current progress, challenges and future perspectives are also discussed. This minireview provides deeper insights into the chemistry of these functional nucleic acids when working with Hg, explains the design ideas of DNAzyme-sensors in each platform, and reveals potential opportunities in developing more advanced DNAzyme sensors for the highly selective and sensitive recognition of Hg. Environmental Implication: Mercury is one of the most toxic metallic contaminants due to its high toxicity, non-biodegradability, and serious human health risks when accumulated in the body. In the recent decade, intensive studies have focused on exploring mercury sensors by combining DNAzymes with various sensing methods, paving a promising avenue to gain ultra-high sensitivity and selectivity. However, so far, no review has introduced the recent advances on DNAzyme-based sensors for mercury detection in a critical way. In this review, we comprehensively summarized the studies on DNAzyme-based sensors for mercury detection using various sensing techniques including optical, electrochemical and other sensing methods.