Brian attended the unveiling ceremony on Thursday (01Sep16) at the unassuming terraced house in Feltham, England.
A blue English Heritage plaque, used to mark buildings of cultural or historical significance, now adorns the property where five decades ago Brian used to listen to records with his bandmate.
"This is very strange for me you know, it's about 50 years ago that I met Freddie and I came here to meet his mum and dad and hang out with him," the musician said during the ceremony. "The last thing on Earth we would have thought would be that I would be here at this point commemorating a blue plaque to my friend's memory."
However Brian expressed his sorrow the Queen frontman was not still around to make music, having died in 1991 aged 45 from an AIDS related illness.
"It's a very happy occasion, but with a tinge of sadness because he should be here, he should be still around creating," he added.
Freddie, born Farrokh Bulsara, moved to the house aged 17 in 1964 after his family came to Britain from Zanzibar, Tanzania.
The singer met both Brian and Queen drummer Roger Taylor while living at the house, with the trio hitting it off over a love of Jimi Hendrix.
"We spent most of the day appreciating and analysing in intimate detail the way that Jimi Hendrix had put his recordings together," Brian told the BBC.
The blue plaque was unveiled ahead of what would have been the late rocker's 70th birthday, on 5 September (16).