The Republic, who play in the third tier of the United States' football pyramid, claim they are "built for MLS" and hailed the Kings' investment as a significant step towards securing a berth in the top-tier of their sport in the USA.
For the Kings, the decision is about making their hometown a "world-class city".
"This is another major step in revitalizing downtown and making the entire Sacramento region an even better place to work, live and play," Kings owner and chairman Vivek Ranadive said in a statement.
"We're committed to our vision of making Sacramento the world-class city its citizens deserve."
The Kings' financial contribution to the Republic comes after the two sporting teams met with the MLS in August to discuss possible expansion to Sacramento for the USA's top football division.
"We shared early on our desire to have the Sacramento Kings as a part of our efforts to secure MLS for Sacramento," the Republic's lead investor Kevin Nagle said.
"Their investment in Republic FC, and support of our bid for expansion rights, is just one more reason why we are 'Built for MLS'."
There is no promotion-relegation in the American football pyramid with the MLS - like most professional American sporting leagues - working on a franchise system.
The MLS currently has 20 clubs with Atlanta and Los Angeles set to add new teams in 2017, while David Beckham's Miami organisation are also expected to join if they can find an appropriate stadium solution.
The Republic started playing in the third-tier USL Pro in 2014, finishing second behind Orlando City - who have moved into the MLS this year - in the regular season, before defeating the Harrisburg City Islanders 2-0 to claim the championship via the play-offs in their debut campaign.
The Republic had an average attendance of 11,293 last year - the highest in USL Pro history - while they also set a new record crowd figure of 29,231 in their debut home match.
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