Low-voltage pentacene-based organic thin-film transistors (OTFTs) have been fabricated using the high-κ organometallic lanthanide complex, Tb(tta)3L2NR (tta = 2-thenoyltrifluoroacetonate, L2NR = (−)-4, 5-pinene bipyridine) as the gate dielectric material. The optimized gate insulator exhibits a low leakage current density of < 10− 7 A cm− 2 under bias voltage of − 5 V, a smooth surface with RMS of about 0.40 nm, a high capacitance of 43 nF cm− 2 and an equivalent κ value of 7. The obtained OTFTs show high electric performance with carrier mobility of 0.20 cm² V− 1 s− 1, on/off ratio of 4 × 10⁵, threshold voltage of − 0.6 V, and subthreshold slope of 0.7 V dec− 1 when operated at − 5 V. The results demonstrate the organometallic lanthanide complex is a promising candidate as gate insulator for low-voltage OTFTs.