Magnetars are strongly magnetized, isolated neutron stars with magnetic fields up to around 10  gauss, luminosities of approximately 10 -10  ergs per second and rotation periods of about 0.3-12.0 s. Very energetic giant flares from galactic magnetars (peak luminosities of 10 -10  ergs per second, lasting approximately 0.1 s) have been detected in hard X-rays and soft γ-rays , and only one has been detected from outside our galaxy . During such giant flares, quasi-periodic oscillations (QPOs) with low (less than 150 hertz) and high (greater than 500 hertz) frequencies have been observed , but their statistical significance has been questioned . High-frequency QPOs have been seen only during the tail phase of the flare . Here we report the observation of two broad QPOs at approximately 2,132 hertz and 4,250 hertz in the main peak of a giant γ-ray flare in the direction of the NGC 253 galaxy , disappearing after 3.5 milliseconds. The flare was detected on 15 April 2020 by the Atmosphere-Space Interactions Monitor instrument aboard the International Space Station, which was the only instrument that recorded the main burst phase (0.8-3.2 milliseconds) in the full energy range (50 × 10 to 40 × 10  electronvolts) without suffering from saturation effects such as deadtime and pile-up. Along with sudden spectral variations, these extremely high-frequency oscillations in the burst peak are a crucial component that will aid our understanding of magnetar giant flares.