DNA self-assembly has created various nanostructured probes deployed in biosensors, whereas their direct charge contribution to sensitive bioassays remains elusive. Here, we report a supercharged tetrahedral DNA nanolabel-based electrochemical (eTDN) sensor for ultrasensitive detection of exosomal microRNAs (Exo-miRs). By using an "assembly before testing" strategy, there is high-efficiency recognition between the target Exo-miR and self-assembled TDN probe in a homogenous solution relative to surface-based hybridization. The TDN-miR complex can be further bridged specifically to form a stable sandwich construct with the surface-confined capture sequence via the base-stacking effect. The intrinsic supercharges of the TDN can adsorb numerous electroactive molecules in stoichiometry, which largely enhances the detection sensitivity, particularly by using electroneutral peptide nucleic acid instead of DNA probes to minimize the background signal. Using this approach, the eTDN sensor achieves a high sensitivity (34 aM), high specificity (against the single mismatch), and high selectivity (in serum). Furthermore, this ultrasensitive sensor provides a conjugation-free, non-enzymatic Exo-miR detection in blood and accurately distinguishes the breast cancer patients from normal individuals, showing to be a promising tool in the early diagnosis of malignant tumors.