The rare martens are now thought to be well established and breeding successfully in the New Forest.
The illusory cat-sized member of the weasel family was previously thought to have survived only largely in northern England.
As part of a long-term study, hidden cameras showed young martens exploring and playing together.
Marcus Ward of the Wild New Forest conservation group said the footage was “amazing”.
The study began after several sightings of the protected species in Hampshire National Park, with conservationists aiming to determine if and how martens were recolonising the area.
More than 100 video clips have been captured in 2022, with footage of juvenile martens providing further evidence of successful breeding.
The new clips also confirm initial findings that martens prefer to use fallen trees and branches to navigate through the forest floor and cross streams and wetlands.
The creatures sleep and make lairs high in trees and usually only come out at night to hunt.
Mr Ward said it was “incredibly rare” to catch a glimpse of a marten.
“This video gives us a unique insight into their hidden world and an opportunity to really advance our understanding of these special creatures and how best to support them.”
The marten study is being conducted by Wild New Forest together with Forestry England, The New Forest Study Group, Hampshire and the Isle of Wight Wildlife Trust.
Leanne Sargeant, senior ecologist for Forestry England, said: “The survey makes it clear they are establishing themselves in a wide range of areas and breeding successfully.
“We hope learning more about their development will help us support these rare creatures and inform reintroduction projects elsewhere.”
A member of the weasel family, the marten prefers well-wooded areas with abundant cover and feeds largely on small rodents, birds, insects and fruit.
They were nearly extinct from England by 1900 due to habitat loss and hunting for their fur.
Only a small and fragmented population remains mostly in northern England, Scotland and parts of Wales.
Each marten is chestnut brown in color but each has a uniquely shaped bib – a pale yellow section of fur on the chin and throat – making it possible to identify and record each individual.
Follow BBC South on Facebook, Chirpingor Instagram. Submit your story ideas to firstname.lastname@example.org.