Folic acid (which is converted to folate (i.e., vitamin B9) in the body) is a water-soluble molecule that is constantly lost and reintegrated daily, and physiological folate deficiencies can cause degenerative conditions, such as megaloblastic anaemia and neural tube defects. Herein, we report the one-step solid-state synthesis of NbFeO4 nanoparticles, which were used to prepare an electrochemical transistor for the highly selective, nanomolar detection of folic acid (FA) in human blood serum samples. Raman spectra and X-ray diffraction (XRD) patterns revealed the combination of monoclinic and orthorhombic phase formations with NbO6 octahedral sites. The NbFeO4-based electrochemical transistor (ECT) exhibits an excellent sensitivity of 0.009 nM cm−2 in the linear range of 1-100 nM as well as a low limit of detection (LOD) of 0.66 nM at an applied potential of 0.6 V. In addition, this sensor shows high selectivity towards FA in the presence of interfering species such as glucose (GS), ascorbic acid (AA), urea and uric acid (UA). The outstanding performance of the sensor can be ascribed to the presence of NbO6 octahedral sites in the NbFeO4 structure which interact with folate (the oxidized form of FA) through Nb4+/Nb5+ niobium states, resulting in an increase in the oxidation peak current. This novel inorganic electrochemical transistor platform can be used to develop high performance, non-enzymatic electrochemical transistors for a wide range of bioanalytical applications. © 2023 The Royal Society of Chemistry.