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Great efforts have been made to understand the impacts of the 1997 Indonesian wildfires on land ecology and human health, yet its influences on marine environments are poorly understood owing to sparse observations. Here we present weekly to monthly resolution coral δ13C records across the South China Sea in conjunction with a regional ocean general circulation model to study the effects of these cataclysmic wildfires on nearby oceans. Large and persistent negative δ13C excursions were found in corals from the interior South China Sea several months after the wildfires. This delay indicates that the 13C‐depleted ash fallout was distributed by ocean currents rather than direct atmospheric transport, which is supported by the results of our numerical model. Thus, the impacts of tropical wildfires on marine ecology may last for 1 year and extend thousands of kilometers from the burning center. Plain Language Summary: Tropical wildfires, intensified by extreme weather events and land use change, have continued to increase during this century, causing serious damage to the rainforests and local economies as well as human suffering and death. The threats of tropical wildfires to nearby oceans have been given little attention and are largely unknown because of limited observations. Here we use multiple coral records with weekly to monthly resolution from five sites near and around the South China Sea and a computer model to study the influences of the 1997 Indonesian wildfires on the ocean. We find a previously unrecognized ash fallout signal recorded in the corals in the interior of the South China Sea. Our model results indicate that the penetration of ash fallout into the South China Sea occurred through ocean currents rather than direct atmospheric transport. This study shows that the impacts of tropical wildfires on oceans may last for 1 year and can extend for thousands of kilometers from the source. We also show the potential for using coral records with computer modeling to identify and study the frequency and extent of past megafires. Key Points: A large negative excursion in δ13C values during summer–winter 1998 was detected in corals from the interior of the South China SeaModel shows that ash fallout from the 1997 Indonesian wildfires distributed by currents was responsible for the negative δ13C excursionNetworks of coral δ13C have great potential to record past pantropical wildfires [ABSTRACT FROM AUTHOR]