Red supergiants are the most common final evolutionary stage of stars that have initial masses between 8 and 35 times that of the Sun(1). During this stage, which lasts roughly 100,000 years(1), red supergiants experience substantial mass loss. However, the mechanism for this mass loss is unknown(2). Mass loss may affect the evolutionary path, collapse and future supernova light curve(3) of a red supergiant, and its ultimate fate as either a neutron star or a black hole(4). From November 2019 to March 2020, Betelgeuse-the second-closest red supergiant to Earth (roughly 220 parsecs, or 724 light years, away)(5,6)-experienced a historic dimming of its visible brightness. Usually having an apparent magnitude between 0.1 and 1.0, its visual brightness decreased to 1.614 +/- 0.008 magnitudes around 7-13 February 2020(7)-an event referred to as Betelgeuse's Great Dimming. Here we report high-angular-resolution observations showing that the southern hemisphere of Betelgeuse was ten times darker than usual in the visible spectrum during its Great Dimming. Observations and modelling support a scenario in which a dust clump formed recently in the vicinity of the star, owing to a local temperature decrease in a cool patch that appeared on the photosphere. The directly imaged brightness variations of Betelgeuse evolved on a timescale of weeks. Our findings suggest that a component of mass loss from red supergiants(8) is inhomogeneous, linked to a very contrasted and rapidly changing photosphere.