Electrochemiluminescence (ECL) combines electrochemical triggering with optical readout, giving it high sensitivity, low background, and good spatiotemporal control. DNA nanostructures are attractive in this context because their sequences, shapes, and functional groups can be programmed with nanometer precision. This review surveys how DNA nanotechnology has been used to improve ECL biosensors. We first outline annihilation and co-reactant ECL mechanisms, and then use a structure-activity framework to compare 1D, 2D, and 3D DNA architectures in terms of interfacial ordering, donor-acceptor distance control, and nanoconfinement. We next examine amplification strategies, including HCR, CHA, RCA, and DNA walkers, and discuss DNAzymes, logic circuits, and CRISPR-Cas cascades as emerging functional modules. Applications are grouped by analyte class, covering pathogen nucleic acids, microRNAs, circulating tumor DNA, protein biomarkers, small molecules, metal ions, and intracellular markers. Finally, we highlight challenges related to stability in biological fluids, scalable fabrication, in vivo compatibility, and the sensitivity-dynamic range trade-off, and suggest future directions such as machine-learning-guided DNA nanoengineering, adaptive single-cell ECL platforms, and point-of-care integration.