The analysis of ribonucleic acid (RNA) plays an important role in the early diagnosis of diseases and will greatly benefit patients with a higher cure rate. However, the low abundance of RNA in physiological environments requires ultrahigh sensitivity of a detection technology. Here, we construct a portable and smart-phone-controlled biosensing platform based on disposable organic electrochemical transistors for ultrasensitive analysis of microRNA (miRNA) biomarkers within 1 h. Due to their inherent amplification function, the devices can detect miRNA cancer biomarkers from little-volume solutions with concentrations down to 10-14 M. The devices can distinguish blood miRNA expression levels at different cancer stages using a 4T1 mouse tumor model. The technique for ultrasensitive and fast detection of RNA biomarkers with high selectivity opens a window for mobile diagnosis of various diseases with low cost.