Asteroid discoveries are essential for planetary-defence efforts aiming to prevent impacts with Earth1, including the more frequent2 megaton explosions from decametre impactors3-6. Although large asteroids (≥100 kilometres) have remained in the main belt since their formation7, small asteroids are commonly transported to the near-Earth object (NEO) population8,9. However, owing to the lack of direct observational constraints, their size-frequency distribution (SFD)-which informs our understanding of the NEOs and the delivery of meteorite samples to Earth-varies substantially among models10-14. Here we report 138 detections of some of the smallest asteroids (≳10 metres) ever observed in the main belt, which were enabled by JWST's infrared capabilities covering the emission peaks of the asteroids15 and synthetic tracking techniques16-18. Despite small orbital arcs, we constrain the distances and phase angles of the objects using known asteroids as proxies, allowing us to derive sizes through radiometric techniques. Their SFD shows a break at about 100 metres (debiased cumulative slopes of q = -2.66 ± 0.60 and -0.97 ± 0.14 for diameters smaller and larger than roughly 100 metres, respectively), suggestive of a population driven by collisional cascade. These asteroids were sampled from several asteroid families-most probably Nysa, Polana and Massalia-according to the geometry of pointings considered here. Through further long-stare infrared observations, JWST is poised to serendipitously detect thousands of decametre-scale asteroids across the sky, examining individual asteroid families19 and the source regions of meteorites13,14 'in situ'.

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