The embassy in the capital, Honiara, is starting small, with a charge d'affaires, a couple of State Department staff and a handful of local employees.
The US previously operated an embassy in the Solomon Islands for five years before closing it in 1993 as part of a global reduction in diplomatic posts after the end of the Cold War.
But China's bold moves in the region have the US seeking to increase its engagement in a number of ways, such as by donating COVID-19 vaccines, bringing back Peace Corps volunteers to several island nations, and investing in forestry and tourism projects.
"The opening of the embassy builds on our efforts not only to place more diplomatic personnel throughout the region, but also to engage further with our Pacific neighbours, connect United States programs and resources with needs on the ground, and build people-to-people ties," Secretary of State Antony Blinken said on Thursday.
The opening comes as Fijian Prime Minister Sitiveni Rabuka appears to be reassessing some aspects of his nation's engagement with China.
Rabuka said last week he planned to end a police training and exchange agreement with China.
The US State Department notified lawmakers early last year that China's growing influence in the region made reopening the Solomon Islands embassy a priority.
Since then, the Solomons has signed a security pact with China, raising fears of a military build-up in the region, and the US has countered by sending several high-level delegations.
The Solomon Islands switched allegiance from the self-ruled island of Taiwan to Beijing in 2019, threatening the close ties with the US that date to World War II.
A senior State Department official who insisted on anonymity to brief the media said the US had been encouraged by the Solomons' commitment to continue working with traditional security partners such as Australia and the US, but remained concerned about the secrecy surrounding the security agreement with China.
The official said the US had yet to have deep conversations with the new Fijian leadership so it was too early to tell if the move on policing signalled a change in direction for Fiji on China.
The Fijian government did not immediately respond to a request for comment.
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