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Description
High-frequency quasi-periodic oscillations (QPOs) in black hole X-ray binaries, typically in the 60–450 Hz range, are associated with processes near the innermost stable circular orbit and scale directly with black hole mass. For supermassive black holes, analogous oscillations are expected in the sub-millihertz regime and were first detected in the narrow-line Seyfert 1 galaxy RE J1034+396. These detections extend the mass–frequency scaling relation from stellar mass to supermassive accreting systems. However, the intermediate-mass range (10⁴–10⁶ M⊙), where QPOs are expected in the millihertz regime, remains underrepresented. Here we present observations of the lensed active galactic nucleus ULAS J081621.49+213442.65 at z ≈ 7.46, an intermediate mass black hole candidate magnified by a foreground galaxy by a factor exceeding 1000, making this intrinsically faint source observable. We detect a stable quasi-periodic signal in the rest-frame UV light curves of the object, at millihertz frequencies, persisting across multiple epochs. The frequency of the QPO implies a black hole mass of a few ×10⁴ M⊙, providing the first compelling evidence for long-sought intermediate-mass black hole progenitors of supermassive black holes at high redshift.