The court said no new information had emerged in a long-standing legal deadlock between Assange and Swedish authorities, who want to question him over allegations he committed rape in 2010.
In a statement, the court said: "The court of appeal shares the assessment of the district court that Julian Assange is still suspected on probable cause of rape."
The Wikileaks chief has avoided potential extradition to Sweden by taking up residence in Ecuador's embassy in London, where he has now been based since 2012.
He is also wanted by US authorities in connection with the criminal investigation surrounding Wikileaks, which Assange launched in 2006.
In February, the Metropolitan Police said it will still arrest Assange if he leaves the embassy, despite reports that suggested a UN panel had ruled in his favour. The Met removed its 24-hour guard outside the embassy last October.
Wikileaks has released "more classified intelligence documents than the rest of the world press combined", the group says on its website.