<p>Structure on subgalactic scales provides important tests of galaxy formation models and the nature of dark matter. However, such objects are typically too faint to provide robust mass constraints. Here we report the discovery of an extremely low-mass object detected by means of its gravitational perturbation to a thin lensed arc observed with milli-arcsecond-resolution very long baseline interferometry. The object was identified using a non-parametric gravitational imaging technique and confirmed using independent parametric modelling. It contains a mass of <i>m</i>₈₀ = (1.13 ± 0.04) × 10⁶ <i>M</i>_⊙ within a projected radius of 80 pc at an assumed redshift of 0.881. This detection is extremely robust and precise, with a statistical significance of 26<i>σ</i>, a 3.3% fractional uncertainty on <i>m</i>₈₀ and an astrometric uncertainty of 194 μas. This is the lowest-mass object known to us, by two orders of magnitude, to be detected at a cosmological distance by its gravitational effect. This work demonstrates the observational feasibility of using gravitational imaging to probe the million-solar-mass regime far beyond our local Universe.</p>