Ms Onassis, 20, the only heir to her grandfather's fortune, married Alvaro Affonso "Doda" de Miranda Neto, 32, in a Sao Paulo mansion under heavy security to deter onlookers and paparazzi.
Security was so tight that guests were not allowed to bring mobile phones, for fear they could take pictures.
One of the few pictures taken by paparazzi from helicopters is a fuzzy shot taken through the mansion's glass ceiling.
The couple is seen at a large table and surrounded by guests.
Ms Onassis is seen wearing a white Valentino dress, her dyed blond hair tumbling down her bare shoulders.
According to local news media the couple will be living in a 1,000-square metre apartment in Sao Paulo near the residence of Doda's parents.
Doda is a bronze medalist from the 2004 Sydney Olympics and the son of one of Brazil's top industrialists.
He has a six-year-old daughter from his previous relationship with a Brazilian fashion model.
Athina met Doda two years ago in Belgium when she was taking riding classes from a Brazilian expert.
Her grandfather, Aristotle Onassis, was one of the world's wealthiest men and also the second husband of the late Jacqueline Kennedy Onassis, widow of assassinated US president John Kennedy.