Charlie Rowley, 45, was released from Salisbury hospital on Friday, three weeks after falling ill alongside his partner Dawn Sturgess, who later died.
“I’m very surprised because when I first saw him I thought he would be there for months,” said Matthew Rowley.
“Obviously he’s stronger than I thought.”
Mr Rowley said his brother’s recollection of what happened was “very vague”, but added: “He definitely said to me that they found this bottle of something and Dawn sprayed it on her wrists and that he picked it up and broke it somehow – and that’s how he got it on his hands.”
He said his brother could not remember where they had found the bottle.
Charlie Rowley and Ms Sturgess, 44, collapsed on 30 June after coming into contact with the nerve agent.
The couple were exposed in Amesbury, near Salisbury, where Russian former spy Sergei Skripal and his daughter Yulia were also poisoned by novichok in March.
Mr Rowley and the Skripals survived, but Ms Sturgess died on 8 July. The mother-of-three’s death is being treated as murder.
Lorna Wilkinson, from Salisbury NHS Foundation Trust, confirmed Mr Rowley had left hospital, saying: “Charlie has been through an appalling experience most of us could never imagine.
“Today is a very welcome milestone in his recovery and all of us here at Salisbury Hospital wish him well as he continues to get better.”
Public Health England said Mr Rowley’s release does not create a risk to the public.
PHE medical director Paul Cosford reiterated its previous advice: “Do not pick up any items such as syringes, needles, cosmetics or similar objects made of materials such as metal, plastic or glass. If you didn’t drop it, then don’t pick it up.”
Last week, the Metropolitan Police confirmed they had found a small bottle believed to be the source of the contamination.
Detectives are investigating the possibility – described by Mr Rowley’s brother – that Ms Sturgess tested the spray on her face and hands, believing it to be perfume.
A post-mortem got under way this week to try to establish the precise cause of her death.
The results may help inform the process to establish conclusively whether the novichok was from the same batch that was used in Salisbury to poison the Skripals.
Police believe they have identified the suspected perpetrators of the novichok attack on the Skripals, it was reported this week.
Britain has blamed Russia for the novichok poisonings of the Skripals and the accidental poisoning of Mr Rowley and Ms Sturgess – with Russia denying the charges.