The largest volcanic eruption of this century, which was submarine, led to a dramatic phytoplankton bloom north of the island of Tongatapu, in the Kingdom of Tonga. In the absence of shipboard observations, we reconstructed the dynamics of this event by using a suite of satellite observations. Two independent bio-optical approaches confirmed that the phytoplankton bloom was a robust observation and not an optical artifact due to volcanogenic material. Furthermore, the timing, size, and position of the phytoplankton bloom suggest that plankton growth was primarily stimulated by nutrients released from volcanic ash rather than by nutrients upwelled through submarine volcanic activity. The appearance of a large region with high chlorophyll a concentrations less than 48 hours after the largest eruptive phase indicates a fast ecosystem response to nutrient fertilization. However, net phytoplankton growth probably initiated before the main eruption, when weaker volcanism had already fertilized the ocean.