We report the design and fabrication of a “signal-on” electrochemical aptamer-based sensor for detection of cadmium (Cd(II)). A 36-base thiolated and methylene blue (MB)-modified aptamer was used in the fabrication of the sensor. In the absence of Cd(II), the surface-immobilized aptamer probe is presumed to be partially folded. Upon binding to Cd(II), the aptamer probe changes its confirmation and flexibility, which alters the electron transfer kinetics between the tethered MB label and the underlying electrode, resulting in an increase in the MB signal. Parameters affecting the detection of Cd(II) were systematically optimized and the electrochemical performance of the sensor was assessed using both alternating current voltammetry and cyclic voltammetry. The sensor has demonstrated good selectivity and can be employed directly in complex samples such as 50% saliva. Furthermore, it has a fast-response time; the analysis can be completed in ~1min. Similar to other folding- and dynamics-based electrochemical biosensors, it is also regenerable and reusable. With further optimization, it could find applications in the analysis of Cd(II) in biological and environmental samples. Display omitted •A DNA aptamer is used in the design of a “signal-on” electrochemical Cd(II) sensor.•Both AC voltammetry and cyclic voltammetry can be used for sensor interrogation.•The sensor is reagentless and can be reused for three times.•The sensor's response to the target is fast (~1min).•The sensor is selective and can function well in complex samples.