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Accreting white dwarfs are often found in close binary systems with orbitalperiods ranging from tens of minutes to several hours. In most cases, theaccretion process is relatively steady, with significant modulations onlyoccurring on time-scales of ~days or longer. Here, we report the discovery ofabrupt drops in the optical luminosity of the accreting white dwarf binarysystem TW Pictoris by factors up to 3.5 on time-scales as short as 30 minutes.The optical light curve of this binary system obtained by the TransitingExoplanet Survey Satellite (TESS) clearly displays fast switches between twodistinct intensity modes that likely track the changing mass accretion rateonto the white dwarf. In the low mode, the system also displaysmagnetically-gated accretion bursts, which implies that a weak magnetic fieldof the white dwarf truncates the inner disk at the co-rotation radius in thismode. The properties of the mode switching observed in TW Pictoris appearanalogous to those observed in transitional millisecond pulsars, where similartransitions occur, although on timescales of ~tens of seconds. Our discoveryestablishes a previously unrecognised phenomenon in accreting white dwarfs andsuggests a tight link to the physics governing magnetic accretion onto neutronstars.

An accreting white dwarf displaying fast transitional mode switching