The caricature by French artist Xavier Ghazi portrays the WikiLeaks founder with his trousers around his ankles, urinating into a top hat with the US flag on it.
The 60-year-old artist says he is overjoyed to win the prize for the fourth time.
Ghazi said he had first thought of calling his painting Pissing Off The Empire.
‘‘Having a leak in Uncle Sam’s hat is pissing off the empire,’’ he told reporters.
‘‘It’s not as much about the US as it is about global power and instruments of domination.’’
Mr Ghazi says he chose Mr Assange because he agrees with the philosophy behind WikiLeaks.
Organisers claim the Bald Archy, which is in its 18th year, is judged by a sulphur-crested cockatoo called Maude.