The court ruled that United States had given a package of assurances to Britain about the conditions of Assange's detention.
The United States has won an appeal in London's High Court to have Wikileaks founder Julian Assange extradited to face criminal charges, including breaking a spying law and conspiring to hack government computers.
"The court allows the appeal," Judge Timothy Holroyde said on Friday.
He said the United States had given a package of assurances to Britain about the conditions of Assange's detention.
Assange's partner, Stella Moris, said they would "appeal this decision at the earliest possible moment".
US authorities accuse Australian-born Assange, 50, of 18 counts relating to Wikileaks’ release of vast troves of confidential US military records and diplomatic cables which they said had put lives in danger.
The United States was appealing against a January 4 ruling by a London District Judge that Assange should not be extradited because he would likely commit suicide in a US prison.
A two-day hearing was held in October where US lawyers argued that the original judge had not given sufficient weight to other expert testimony about Assange's mental state.
They also sought to assure the court that he would not be held in punishing isolation at a federal supermax prison, and would receive appropriate treatment.
Two appeal judges at the High Court in London accepted US assurances that Assange would not face the strictest measures before any trial or after conviction.
"That conclusion is sufficient to determine this appeal in the USA's favour," they said.
The case will now go back to the lower court to be considered again.
If extradited, tried and convicted, Assange could be jailed for up to 175 years, although the exact sentence is difficult to estimate and could be shorter.
Assange has been in custody since 2019, despite having served a previous sentence for breaching bail conditions in a separate case.