Ovarian cancer is a significant health concern among the female vulnerable populations due to delayed diagnosis. Point-of-care devices that can be utilized for mass screening of the population may improve patient survival through early diagnosis. In this context, we report an ultrasensitive DNA-based nano-biosensor for the quantification of cancer biomarker miRNA-21 using a label-free sensing technique. Magnetite selenium ( ) nanocomposite was prepared by co-precipitation method followed by characterization using TEM, EDX, FTIR, VSM, and XRD. The average size of the nanomaterial was found to be 20 nm with a low polydispersity index of 0.2. The biosensor was fabricated by immobilizing a single-stranded DNA probe specific to miRNA-21 onto the nanocomposite modified electrode surface. At the working electrode/electrolyte interface, the binding of the DNA probe results in decreased charge transfer impedance. The developed magnetite-based impedimetric biosensor demonstrated a linear variation of resistance to charge transfer with miRNA-21 concentration (75 zM to 0.751 pM). The ultrasensitive low limits of detection (75 zM), the sensitivity of 1774.34 / g/ L/ and high selectivity over other biomarkers (miRNAs-miRNA-30e, miRNA-143, miRNA-204, DNA, protein, and cocktail of all makes it suitable for early diagnosis. Furthermore, owing to miRNA-21 prognostic as well as diagnostic value, the magnetite-based miRNA-21 impedimetric biosensor offers the promise of a sensitive consumer-centric device for mass screening of ovarian cancer.