Exosomes are nanoscale membrane vesicles actively released by cells and play an important role in the diagnosis of cancer-related diseases. However, it is challenging to efficiently enrich exosomes from extracellular fluids. In this work, we used DNA nanostructures as "nanoweights" during centrifugation to facilitate the enrichment of cancerous exosomes in human serum. Two different DNA tetrahedral nanostructures (DTNs), each carrying a specific aptamer for exosome biomarker recognition, were incubated with clinical samples simultaneously. One DTN triggered the cross-linking of multiple target exosomes and, therefore, enabled low-speed and fast centrifugation for enrichment. The other DTN further narrowed down the target exosome subtype and initiated a hybridization chain reaction (HCR) for sensitive signal amplification. The method enabled the detection of 1.8 × 102 MCF-7-derived exosomes per microliter and 5.6 × 102 HepG2-derived exosomes per microliter, with 1000-fold higher sensitivity than conventional ELISA and 10-fold higher sensitivity than some recently reported fluorescence assays. Besides, the dual-aptamer system simultaneously recognized multiple surface proteins, eliminating the interference risk from free proteins. Thus, this easy-to-operate method can enrich exosomes with excellent specificity and sensitivity and therefore will be appealing in biomedical research and clinical diagnosis.