Clarach landed in Glaslyn, Snowdonia, on Tuesday.
Welshpool-based Montgomeryshire Wildlife Trust said she was the first chick from the Dyfi Osprey Project to be seen again as an adult.
The organisation said: "We've witnessed another milestone in the developing story of Welsh osprey recolonisation."
Clarach was born in 2013 and migrated in September that year.
Emyr Evans, Dyfi Osprey Project manager, said: "When they're chicks they migrate, mostly to Africa, and they stay there for two or three years. Then they come back, but two thirds usually die on the journey.
"It's an enormous task and challenge to try and find the birds, but what happened yesterday is that one of our adults was re-sighted as an adult back in Wales. This is the first time any of the Dyfi birds have been re-sighted back as adults."
It is not known if Clarach came back to Wales last year, but was not recorded.
A blog post on the Dyfi Osprey Project's website called this "the promised land" and apologised to any visitors on Tuesday who "may have encountered crying, shouting and hysterical volunteers".
An osprey from a Powys breeding programme has returned to its nest, becoming its first to be re-sighted in the UK, a wildlife trust has said.