MicroRNAs (miRs) have emerged as important clinical biomarkers with both diagnostic and prognostic value for relevant diseases, such as cancer. MiRs pose unique challenges for detection and are currently detected by northern blotting, real-time PCR, and microarray techniques. These expensive, complicated, and time-consuming techniques are not feasible for on-site miR determination. In this study, amperometric magnetobiosensors involving RNA-binding viral protein p19 as a selective biorecognition element were developed for miR quantification. The p19-based magnetosensors were able to detect 0.4 fmol of a synthetic target and endogenous miR-21 (selected as a model for its role in a wide variety of cancers) in only 2 h in total RNA extracted from cancer cells and human breast-tumor specimens without PCR amplification and sample preprocessing. These results open up formidable perspectives for the diagnosis and prognosis of human cancers and for drug-discovery programs.