In-plane-gatefield-effect transistors (IPGFETs) offer an innovative device architecture in which the channel conductivity is modulated by the electric field from the 2D electron gas in the two adjacent in-plane gates, isolated by etched trenches. The planar nature of the gate electrode yields a huge reduction in parasitic gate capacitance, which can lead to much higher frequency. Moreover, the fabrication process for these devices is extremely simple and with inherently self-aligned gates. Here, we combine for the first time the promising architecture of IPGFETs with the exceptional properties of III-Nitrides, such as large carrier density and breakdown field, to reveal their enormous potential for high-power RF devices. AlGaN/GaN IPGFETs demonstrated large drain current up to 1.4 A/mm and transconductance up to 665 mS/mm, which are, respectively, nine times-and five times-larger than the best IPGFETs demonstrated in other semiconductors. These devices presented excellent gate control with ON-OFF ratio up to 10(7) along with ultra-low capacitances down to 0.7 aF, leading to an estimated f(T) up to 0.89 THz. Extremely large breakdown voltage of 500 V was observed despite their nanoscale dimensions, with small leakage current below 1 nA up to 300 V. These results reveal that III-Nitride IPGFETs offer a promising pathway for future terahertz devices delivering large output powers.