Around 100 short-haired bumblebees (Bombus subterraneus) were brought across from Sweden to repopulate areas where it previously thrived in the UK.
Around 50 of the healthiest were released at the RSPB's Dungeness reserve in Kent.
The conservation project to bring the bee back has involved the creation of flower-rich meadows and field margins in the landscape, which have boosted populations of other threatened bumblebees.
The short-haired bumblebee was last recorded in the UK in the Dungeness area in 1988, having declined over the previous 60 years as habitats were lost. It was officially declared extinct in 2000.
After it vanished in this country, small populations clung on in the South Island of New Zealand after being transported there on the first refrigerated lamb boats in the late 19th century to pollinate crops of red clover.
Attempts to transport queen bees back from the other side of the world failed in 2009 when the bees died before the end of their quarantine period.
For the latest attempt, conservationists turned to a healthy population of the bee living in the southern Swedish province of Skane.
Queen bees were captured using bee nets at the end of April for the Natural England-backed project, which is also supported by the RSPB, Hymettus and the Bumblebee Conservation Trust.
The queens were placed in temporary hibernation and brought over to a quarantine facility at the Royal Holloway University in Surrey, where they were checked over before being released.
The conservationists have been working with UK farmers across Romney Marsh and Dungeness over the past three years to create flowering field margins suitable for bees.