In a post on his Instagram page earlier this month, the Foster + Partners founder shared three pictures of himself trying on Rapha garments, including a stream-lined black and silver jacket, and a peaked cap with pink and silver lettering.
The caption reads: "With the @rapha team at the Foster Riverside Studio working on my new range of clothing."
Foster, who ranked at number 13 on the inaugural Dezeen Hot List, has often described cycling as one of his greatest passions.
His new 930-home skyscraper on London's City Road features impressive cycling facilities, including 1,486 bike parking spaces – which equates to one per bedroom – as well as dedicated bike lifts and a bike workshop for residents.
Similarly, back in 2014, Foster unveiled a vision for a network of elevated cycle routes above London's railways, intended to prevent the number of cyclist casualties in the city.
"I believe that cities where you can walk or cycle, rather than drive, are more congenial places in which to live," he said at the time.
The architect has been working with Rapha for more than a year. He featured briefly in a movie about Rapha's head of design Alex Valdman in late 2016, but has otherwise kept details of the partnership under wraps.
Rapha has not yet replied to Dezeen's request for more information – so it is unclear at this stage when the collection will be released.