Dormice are found in Africa, Asia and Europe and are known for their long hibernation periods.
Conservationists believe dormouse populations have collapsed 70% in the last 25 years in the UK, becoming extinct in 14 counties, due to habitat loss.
But the little one found in a tree hopefully has a chance of survival.
The Essex trust said Nicky Payne, its area officer for north west Essex, was working with volunteers when they saw the deflated helium balloon.
When they removed it “they felt the torpid dormouse, who was thankfully unharmed… and relocated the little dormouse into a safe habitat”.