This study reports a hybrid electrokinetic microdevice for rapid concentration and detection of circulating cell-free (cf)DNA. Rapid molecular analysis of cfDNA has the potential to change the current practice of medicine, such as in cancer diagnostics and in monitoring the efficacy of cancer treatments. With a combination of AC electrothermal flow and dielectrophoresis, the hybrid electrokinetic microdevice efficiently concentrates cfDNA from blood plasma and other physiological fluids. In this design, the long-range AC electrothermal flow, which is effective in conductive fluids, drives the cfDNA towards the center of the electrode where dielectrophoretic trapping of the cfDNA occurs. Once the cfDNA is collected at the electrode, the concentration in the blood sample can be quantified by fluorescence analysis with an intercalating dye that binds specifically to double-stranded DNA. The effects of the electrokinetic parameters were elucidated to optimize the design of the device. The device was demonstrated to separate high molecular weight DNA from low molecular weight DNA. Quantitative detection of clinically relevant concentrations of cfDNA was achieved in 10 minutes.