The 20-year-old was tragically found dead six hours later near London Bridge in what police described as a “sheer act of bravery” to rescue the 21-year-old woman who survived.
Unveiling the plaque with Jimi’s grieving parents Michael and Olasunkanmi, Mr Khan said: “Jimi was a hero. His story should be told again and again and again.
“Responding to a woman who needed help, he knew he was risking his life to help another and he paid the ultimate price.
“Strength, selflessness, courage, humility, bravery, all those things are bottled up in this one man Jimi.
“It’s so important this plaque is here. Because his memory and spirit must live on.
“He was the best of us.”
Flowers were left beside the plaque after its installation at Cathedral Square.
Jimi’s brother Bolaji Olubunmi-Adewole said at the ceremony: “We no longer feel sorry.
“Today we are celebrating life now. We are grateful for this recognition of Folajimi. Very big thank you to Living Bankside, to Southwark Cathedral, the Mayor of London for making this happen.”
Jimi was walking with his best pal Bernard Kosia after a night shift across London Bridge just after midnight on April 24 when he heard the desperate cries of the woman in the Thames.
Mr Kosia told an inquest that Jimi told him to stay on land because he could not swim.
The inquest at Inner South London Coroners Court heard Joaquin Garcia, who saw the woman as he changed buses, jumped in first followed by Mr Olubunmi-Adewole five or 10 seconds later.
In a statement read out by the coroner, Mr Kosia said: “The whole time Jimi was saying ‘I have got to save her.’
“He was very adamant about it. He was taking off his clothes saying ‘I have to save her, she is not dying.’
“The woman was struggling to stay afloat. I could hear her voice and she wanted to be rescued.
“There was clear pain in her voice and she was struggling.
“Jimi turned around and told me ‘you can’t swim. This man and I can, and we are going to save this woman.
“They counted out to three and jumped in. Mr Garcia jumped first and then Jimi jumped.
“Then around two minutes later he was shouting my name shouting ‘jump’. I could hear him shouting out and I could not see him anywhere.”
Mr Garcia and the woman were pulled out of the water five minutes later before police helicopters and marine crews began a search for Mr Olubunmi-Adewole, which lasted almost an hour.
Among those paying tribute to the 20-year-old hero in the wake of his death was popstar Dua Lipa, who dedicated one of her awards at the BRIT awards to him.
Mr Olubunmi-Adewole, from Bermondsey, was posthumously put forward for a Royal Humane Society award by City of London Police to honour his “memory and heroism” for his “bravery and selfless actions”.