In a message to followers on the encrypted Telegram messaging app, Robinson wrote: “As I lay in bed early on this Sunday night all I can think is FREE JULIAN ASSANGE.”
The anti-Islam activist, whose real name is Stephen Yaxley-Lennon, was released on Friday but Assange remains imprisoned for breaching bail by fleeing to the Ecuadorian embassy in London in 2012.
The WikiLeaks founder was wanted for questioning over alleged sexual assault in Sweden, which he denies, and could also be extradited to the US on hacking and espionage charges.
Robinson, who characterises himself as an anti-sexual abuse campaigner, said he did not meet Assange in person because he was being held in an isolation unit due to safety concerns.
But in an interview with his former employer Rebel Media, Robinson said Assange’s room in a separate health ward overlooked an exercise yard he used.
He claimed they spent half an hour talking through a window, in one session shortly before his release.
The Independent understands that the layout of HMP Belmarsh makes such conversations possible, but none were formally witnessed or recorded by prison staff.
Some of Robinson’s supporters said they would write letters of support to Assange, after characterising both men as “political prisoners”.
Assange’s representatives could not immediately confirm Robinson’s account.
Since his release, Robinson has also announced his support for the prime minister, and said the 21 “purged” MPs “aren’t actually Conservatives”.
“I actually like Boris Johnson, I think he’s shown himself as a very strong leader,” he said. “The battle that’s commencing is Boris Johnson, Eton-educated, representing the people [over Brexit].”