Work started last August on the new museum now called the Coos History Museum and Maritime Collection.
The museum's rotating exhibits will be based on input from the community.
"It's been a long slog for a lot of people. They've been raising funds for 15 years for this project, and it's just heroic really," said Frank Smoot, the museum's executive director.
Smoot said the staff have photographed and cataloged every piece in the old museum to ensure they preserve the existing collections when the new space is completed.
Tours are already lined up for the new Coos History Museum and Maritime Collection, expected to open in December.