A deal must now be worked out between the two clubs and Freeland regarding the 1.5 million euro buyout of his contract with the Spanish club, which has two years left to run.
"Unicaja player Joel Freeland has informed the club of his decision to end his contract and move to the Portland Trail Blazers of the NBA," said a statement on the Unicaja website.
"After receiving this statement from the British player, Unicaja is now waiting to formalise a definitive agreement between the three parties involved; the player, the Portland Trail Blazers and Unicaja."
Freeland will also need to travel to Portland for a medical, which could affect his availability for Britain's final warm-up games for the Olympics, with the team scheduled to face Portugal in Sheffield on Saturday and Sunday and then the United States in Manchester on Thursday.
He was drafted by Portland in 2006 when aged 19, but the NBA team encouraged him to continue his development in Europe and he first broke through with Gran Canaria in Spain's tough ACB league.
The 6ft 10in 25-year-old made his GB debut in 2006 and established himself as one of the team's leading players with some outstanding displays in the 2011 European Championship.