Photosynthesis is one of the most important biological processes on Earth, providing the main source of bioavailable energy, carbon, and oxygen via the use of sunlight. Despite this importance, the minimum light level sustaining photosynthesis and net growth of primary producers in the global ocean is still unknown. Here, we present measurements from the MOSAiC field campaign in the central Arctic Ocean that reveal the resumption of photosynthetic growth and algal biomass buildup under the ice pack at a daily average irradiance of not more than 0.04 ± 0.02 µmol photons m<jats:sup>−2</jats:sup> s<jats:sup>−1</jats:sup> in late March. This is at least one order of magnitude lower than previous estimates (0.3–5 µmol photons m<jats:sup>−2</jats:sup> s<jats:sup>−1</jats:sup>) and near the theoretical minimum light requirement of photosynthesis (0.01 µmol photons m<jats:sup>−2</jats:sup> s<jats:sup>−1</jats:sup>). Our findings are based on measurements of the temporal development of the under-ice light field and concurrent measurements of both chlorophyll <jats:italic>a</jats:italic> concentrations and potential net primary production underneath the sea ice at 86 °N. Such low light requirements suggest that euphotic zones where photosynthesis can occur in the world’s oceans may extend further in depth and time, with major implications for global productivity estimates.