The incident, which lasted for 45 minutes and was caught on camera, is reported in the Journal of Ethology.
The bizarre event took place on a beach on Marion Island, a sub-Antarctic island that is home to both fur seals and king penguins.
Equally, it might be been an aggressive, predatory act; or even a playful one that turned sexual.
"At first glimpse, we thought the seal was killing the penguin," says Nico de Bruyn, of the Mammal Research Institute at the University of Pretoria, South Africa.
The brazenness of the seal's behaviour left those who saw it in no doubt as to what was happening.