"The new office complex is a symbol of the strength and vibrancy of the US-Taiwan partnership in the 21st century."
Royce is the wife of a long-time Taiwan advocate and House Foreign Affairs Committee chairman, Ed Royce.
Also among the guests of honour were William Moser, principal deputy director of the US Overseas Office, and US Congressman Gregg Harper, co-chairman of the US Congressional Taiwan Caucus.
A smiling Tsai said the new building was a symbol of "more promising" relations between Washington and Taipei.
Ties between Taiwan and mainland China have been strained since Tsai, from the independence-leaning Democratic Progressive Party, took office in 2016.
Beijing considers Taiwan to be a breakaway Chinese province to eventually be reunited with the mainland, by force if necessary.
Tsai has also refused to acknowledge the "1992 consensus" that there is only "one China" and that Taiwan forms part of it.
Beijing says the principle forms the basis of any ties between the mainland and Taiwan.
Beijing has sought to increase political pressure on the island, halting any cross strait dialogue and isolating it internationally by poaching its few overseas diplomatic allies.
China has also stepped up military exercises and air patrols close to Taiwan.
Trump decided not to send cabinet-level officials to the opening ceremony to avoid further antagonising Beijing amid other tensions over trade and China's claims to much of the South China Sea, sources with ties to the US and Chinese governments told the South China Morning Post earlier this month.
Taiwanese politicians, however, hope that more senior US officials such as US Secretary of State Mike Pompeo or national security adviser John Bolton may visit the island this autumn.
This story first appeared in the South China Morning Post and was reproduced with their permission.