The birds of prey were once native to the area it but has not had a breeding pair for more than a century.
Working with Forest Enterprise Scotland, the Roy Dennis Wildlife Foundation has been collecting chicks under licence in north east Scotland.
The birds are being released at the Urdaibai Estuary near Bilbao.
Ospreys visit the estuary during their migrations to west Africa from breeding sites in Scotland.
Alan Campbell, environment ranger with Forest Enterprise Scotland's Aberdeenshire team, said: "This has been a great project to be involved in.
"It feels really good to know that we have helped reintroduce these magnificent birds to another part of the world where they have been struggling to hold on.
"Roy Dennis has been weighing, measuring and ringing osprey chicks on the national forest estate for many years, but over the past five years, when there has been more than one chick in a nest, the larger chick has been selected for translocation."
The chicks are from nests in Moray and Aberdeenshire.
Environment Secretary Roseanna Cunningham said: "The story of Scotland's ospreys is a source of inspiration. It demonstrates what can be achieved when individuals and organisations work together to protect and enhance biodiversity.
"We should never forget the osprey had been wiped out in Scotland, before returning in the 1950s.
"A huge amount of work has been done to protect the species and ospreys are now thriving here."
She added: "I would like to congratulate the Roy Dennis Wildlife Foundation and Forest Enterprise Scotland for this latest success.
"It's heartening to see that other countries are now benefiting from the hard work carried out over many years in Scotland."
Conservationists in Scotland are helping to restore a breeding population of ospreys to the Basque Country of Spain.