ABSTRACT We provide the first record of the giant deep-sea jellyfish Stygiomedusa gigantea (Browne, 1910) from the tropical Southwestern Atlantic, based on a posting made on social media. The image was found during data filtering through the hashtag #aguavivagigante (in English, #giantjellyfish) on Instagram. The jellyfish was seen floating motionless at the surface, presumed dead, off Bahia, Northeast Brazil (12°34’39”S; 38°00’19”W), on August 23rd, 2016. Pygmy killer whales, Feresa attenuata Gray, 1874, were observed in the intermediate area and apparently interacting with the jellyfish. Observations of S. gigantea usually come from submersibles and ROVs. This is the first record extracted from social media, highlighting its potential use to complement information based on traditional scientific methods, and expanding the known geographic distribution of jellyfish. With the growth and development of social media in the coming years, we highlight its potential as a data source from citizen science. We also expect that social media will increasingly contribute to the knowledge of remarkable but poorly known species such as S. gigantea.