The role of volcanogenic halogen-bearing (i.e. chlorine and bromine) compounds in stratospheric ozone chemistry and climate forcing is poorly constrained. While the 1991 eruption of Pinatubo resulted in stratospheric ozone loss, it was due to heterogeneous chemistry on volcanic sulfate aerosols involving chlorine of anthropogenic rather than volcanogenic origin, since co-erupted chlorine was scavenged within the plume. Therefore, it is not known what effect volcanism had on ozone in pre-industrial times, nor what will be its role on future atmospheres with reduced anthropogenic halogens present. By combining petrologic constraints on eruption volatile yields with a global atmospheric chemistry-transport model, we show here that the Bronze-Age 'Minoan' eruption of Santorini Volcano released far more halogens than sulfur and that, even if only 2% of these halogens reached the stratosphere, it would have resulted in strong global ozone depletion. The model predicts reductions in ozone columns of 20 to >90% at Northern high latitudes and an ozone recovery taking up to a decade. Our findings emphasise the significance of volcanic halogens for stratosphere chemistry and suggest that modelling of past and future volcanic impacts on Earth's ozone, climate and ecosystems should systematically consider volcanic halogen emissions in addition to sulfur emissions. Halogens (especially chlorine and bromine) play important roles in the catalytic destruction of atmospheric ozone 1–3. However, the role of 'volcanogenic' halogens in stratospheric ozone chemistry and climate forcing remains poorly constrained. It is commonly believed that most of the halogens in explosive volcanic plumes do not enter the stratosphere, because they are removed by hydrometeors in the trop-osphere 4,5. In contrast, volcanic sulfur emissions are known to play a key role in stratospheric ozone change and climate forcing on annual to decadal timescales 6. As stratospheric sulfate aerosols backscatter solar radiation, they act to cool the Earth's troposphere and surface 7. In addition, the surface of sulfate aerosols provides sites for heterogeneous chemical reactions that activate halogen species which destroy ozone 8,9. In the case of recent large volcanic eruptions (1982 El Chichón eruption, 1991 Pinatubo eruption), the halogen compounds involved in such reactions were sourced from anthropogenic emissions 1,10 (e.g., chlorofluorocarbons). The 1991 eruption of Mount Pinatubo (Philippines) furnished particularly significant insights into the mechanisms, feedbacks and timescales of such processes 1. However, the Pinatubo magma was relatively poor in halogens compared to other volcanic eruptions. In addition, it occurred while the global atmosphere was still loaded with anthropogenic emissions of organic halogens. This calls into question the general applicability of the conclusions derived from Pinatubo observations and ensuing modelling to past or forthcoming eruptions. In particular, it is not known what would happen with a volcanic event with a different halogen yield occurring under pre-industrial atmospheric conditions. In addition to these uncertainties, recent models have re-evaluated the fraction of explosively-emitted halogens crossing the tropopause, suggesting significantly higher values, up to 25%