In the Arctic Ocean, phytoplankton blooms on continental shelves are often limited by light availability, and are therefore thought to be restricted to waters free of sea ice. During July 2011 in the Chukchi Sea, a large phytoplankton bloom was observed beneath fully consolidated pack ice and extended from the ice edge to >100km into the pack. The bloom was composed primarily of diatoms, with biomass reaching 1291mg chlorophyll a m-2 and rates of carbon fixation as high as 3.7gCm-2 d-1. Although the sea ice where the bloom was observed was near 100% concentration and 0.8-1.2m thick, 30-40% of its surface was covered by melt ponds that transmitted 4-fold more light than adjacent areas of bare ice, providing sufficient light for phytoplankton to bloom. Phytoplankton growth rates associated with the under-ice bloom averaged 0.9d-1 and were as high as 1.6d-1. We argue that a thinning sea ice cover with more numerous melt ponds over the past decade has enhanced light penetration through the sea ice into the upper water column, favoring the development of these blooms. These observations, coupled with additional biogeochemical evidence, suggest that phytoplankton blooms are currently widespread on nutrient-rich Arctic continental shelves and that satellite-based estimates of annual primary production in waters where under-ice blooms develop are 10-fold too low. These massive phytoplankton blooms represent a marked shift in our understanding of Arctic marine ecosystems.