Academics and family historians can now access hundreds of thousands of documents and museum artefacts which chart the history of the East Riding.
The archives and local studies section was the first area to open, with the lift providing wheelchair access to the adjacent art gallery for the first time.
The top of the tower, which will give visitors stunning views over central Beverley and the Minster, will open in mid-January.
The exhibition area of the Treasure House, which will accommodate a permanent display of East Riding life through the ages and temporary exhibitions, will open towards the end of January.
But it will be without one of its star exhibits, the hoard of Iron Age weapons found at South Cave in 2002, which is still undergoing conservation.
Buried in earth, the weapons rusted together in the 2,000 years they spent in the earth before they were discovered by three metal detectorists in what was described as a "once in a century" find.
Archives and local studies manager Ian Mason said: "People have said how nice and light and airy the building is and there's a lot more space than the building we have moved from.
"In terms of the facilities we are able to provide and the level of comfort, it is vastly improved."
The huge range of resources, some of which date back to the twelfth century, includes registers of deeds, parish registers and records from some of the East Riding's great estates.
The Treasure House will open 9.30am to 5pm on Mondays, Wednesdays and Fridays; 9.30am to 8pm on Tuesdays and Thursdays, and 9am to 4pm on Saturdays.
For more information
telephone 01482 382790
or email archives.service
@eastriding.gov.uk