Rep. Young Kim, R-Calif, defeated her Democrat opponent, retired fire captain Joe Kerr, in California's 40th Congressional District during Tuesday's general election.
The Associated Press called the race for Kim shortly after 9 p.m. ET on Thursday.
With 74% of the vote counted, Decision Desk HQ showed Kim with 56.6% and Kerr with 43.4%.
Overall, Republicans led 216-204, just two seats shy of securing a majority in the new Congress with 15 seats still to be decided.
Kim's seat, largely based in once-Republican bastion Orange County, had been a target for Democrats.
The district voted for President Joe Biden in 2020.
Vice President Kamala Harris was projected to defeat President-elect Donald Trump in Orange County despite trailing 49.2% to 48.2% with 69% of the vote counted.