AbstractK0.5Na0.5NbO3 (KNN) of 4‐μm average particle size was flash sintered in parallel plate capacitor configuration (PPCC) under 100 V/mm (0.8 mA/mm2 cut‐off) with 10°C/min heating rate in air. Precipitous densification occurred from Tf = 662–670°C (Tf–furnace temperature) in 60 s with 55 mW/mm3 peak power absorption at Tf = 667°C, resulting in 98% dense ceramic of < 5‐μm grains size. No grain growth was observed. A cubic grain morphology was inherited from the starting KNN powder. No sign of liquid phase formation on grain boundaries (GBs) was detected. A 147°C rise above Tf = 667°C due to Joule heating is predicted. At 25°C and 1 kHz, relative permittivity (εr) and dielectric loss (tan(δ)) are 4096 and 2.9%, respectively. From 0.1 to 100 kHz, εr and tan(δ) undergo relaxation, reaching εr = 577 and tan(δ) = 0.2% at 1000 kHz. Variation of εr with temperature revealed Amm2 → P4mm and P4mm → Pm3m transitions at ∼204°C and 409°C, respectively. As per Curie–Weiss analysis, Curie temperature and Curie–Weiss constant are θ = 375°C and C = 3.1 × 10−5°C for the P4mm → Pm3m transition, respectively. The transition is first order as Ttr > θ with a diffusiveness exponent γ = 1.04, indicating normal ferroelectric behavior. The low‐frequency dielectric relaxation is attributed to the space charge in the vicinity of GBs, which is a remnant of flash sintering. Joule heating cannot alone account for the densification in 60 s, which we substantiate within the framework of phase equilibrium and diffusion kinetics. We conjecture that oxygen vacancies start ionizing at Tf = 662°C, couple with the applied electric field via the Lorentz force, and increase their electrochemical potential, causing ultrafast densification. The effects of sample shape, size, and PPCC contributing to high sintered density are also discussed.