The 29-year-old singer – who cancelled two dates at Wembley last month – was pictured at IntoUniversity in London on Saturday.
Volunteer Padrinio Blaque shared a number of pictures with the Grammy award winner as she visited the Kensington site.
He wrote: "I'm still trying to process the fact that I met… ADELE?! THE @Adele whilst volunteering for carnival 2K17"
Adele and volunteer Padrinio Blaque
Volunteer Padrinio Blaque wearing his signed hat
A video was also posted to Twitter showing the moment after the Hello singer autographed Mr Blaque's hat.
In the seven-second clip, Mr Blaque says: "Bombshell! Adele has just signed my hat" with the pair laughing before the singer adds: "You can't really see it."
Adele: A life in pictures
Sat, July 1, 2017
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Adele in pictures
Adele was pictured with Padrinio Blaque at IntoUniversity in London on Saturday
Adele was forced to cancel two performances in London after damaging her vocal cords and told fans she was "heartbroken" and "devastated" by the decision.
The sell-out performances at Wembley Stadium on July 1 and 2 would have seen the multiple Grammy-winner round off her global tour in her home city.