Kuiper belt objects (KBOs) are thought to be remnants of the early Solar System, and their size distribution provides an opportunity to explore the formation and evolution of the outer Solar System1,2,3,4,5. In particular, the size distribution of kilometre-sized (radius = 1–10 km) KBOs represents a signature of initial planetesimal sizes when planets form5. These kilometre-sized KBOs are extremely faint, and it is impossible to detect them directly. Instead, the monitoring of stellar occultation events is one possible way to discover these small KBOs6,7,8,9. However, until now, there has been no observational evidence for occultation events of KBOs with radii of 1–10 km. Here, we report the first detection of a single occultation event candidate by a KBO with a radius of ~1.3 km, which was simultaneously provided by two low-cost small telescopes coupled with commercial complementary metal–oxide–semiconductor cameras. From this detection, we conclude that the surface number density of KBOs with radii exceeding ~1.2 km is ~6 × 105 deg−2. This surface number density favours a theoretical size distribution model with an excess signature at a radius of 1–2 km (ref. 5). If this is a true KBO detection, this implies that planetesimals before their runaway growth phase grew into kilometre-sized objects in the primordial outer Solar System and remain as a major population in the present-day Kuiper belt.