Sweden wants to extradite Assange to answer charges of the alleged rape of one woman and the molestation of another in Stockholm last year. Assange has denied wrongdoing.
In the ruling, the appeal judges rejected key arguments from Assange's legal team. They said Sweden had the right to issue a warrant for Assange, rejected claims that the alleged offense had been inaccurately described, dismissed issues over Sweden's process for instigating criminal inquiries, and ruled that the prosecutors had been proportionate in their actions.
Assange has 14 days to decide whether to appeal to Britain's Supreme Court.
The appeal judges confirmed that Assange would remain on bail, held under virtual house arrest at a friend's country estate in southern England.
The Associated Press contributed to this story.