Mr Assange, 50, married Stella Moris on Wednesday in Belmarsh prison, where he has been held since 2019.
The couple were granted permission to marry last year, and the ceremony was attended by four guests as well as two official witnesses and two guards.
Mr Assange is in prison while US authorities seek to extradite him to face trial on espionage charges.
He is wanted over the publication of thousands of classified documents relating to the Afghanistan and Iraq wars. He denies any wrongdoing.
Earlier this month, the UK Supreme Court refused to allow his latest appeal against extradition.
Ms Moris, a 38-year-old lawyer, was greeted by a crowd of supporters as she left the ceremony in south-east London. They threw confetti and shouted "congratulations" and "free Julian Assange".
She wore a wedding dress designed by Dame Vivienne Westwood - who has campaigned to stop the extradition of Mr Assange.
"I am very happy and very sad. I love Julian with all my heart, and I wish he were here," Ms Moris told the crowd, before describing her husband's detention as "cruel and inhuman".
"The love we have for each other carries us through," she said. "He's the most amazing person."
The couple began a relationship in 2015 and have two children together.