Speaking at the ceremony, President Andris Berzins said it was "more than just a library. It is a modern multi-purpose building, which is both a benefit and a challenge to Riga."
That challenge would consist in finding suitable events and users to fill its cavernous halls, Berzins said.
"Every evening, I find some time to read books," Berzins added.
It was the third in a series of opening events.
A 'chain of book lovers' on January 18 attracted worldwide positive press coverage with bibliophiles forming a 2-kilometer-long chain from the old library premises in Riga's Old Town to the new location on the opposite side of the River Daugava.
A second 'opening' on June 16 when parts of the building were opened to the public attracted headlines of a different sort when more than 25 visitors became stuck in a lift and had to be rescued by fire crews and paramedics.
The design of the vast building dubbed the 'Castle of Light' - despite its pale grey color - has divided opinion. Many love the lines drawn up by American Latvian architect Gunnar Birkert in the 1990s, while others are less enthusiastic.
Critics include Riga Mayor Nils Usakovs who famously likened it to a large supermarket.
The total cost of the project is believed to be in excess of 250 million euros.