The short-haired bumblebee started dying out in Britain in the 1980s and officially became extinct in 2000.
A reintroduction project saw queen bees brought over from Sweden. After two releases of queens at the RSPB's Dungeness reserve in Kent, offspring worker bees have been recorded there for the first time.
Short-haired bumblebees were once widespread across the south of England but declined as their wildflower rich grasslands disappeared.
Nikki Gammans, who leads the project, said: "This is a milestone for the project and a real victory for conservation. We now have proof that this bumblebee has nested and hatched young and we hope it is on the way to become a self-supporting wild species in the UK".