Republican Party officials have expressed concern that GOP turnout will suffer without a nominee in the general election.
Cox is battling fellow Republican Travis Allen, and Democrats Antonio Villaraigosa, John Chiang and Delaine Eastin to secure a spot in the general election. Democrat Gavin Newsom is seen as the front-runner and is likely to advance to the November election.
Trump's endorsement follows the support of California House Republicans, including Majority Leader Kevin McCarthy, who came out behind Cox earlier this month.
University of California San Diego political science professor Thad Kousser said Trump was likely following the advice of McCarthy, a Bakersfield Republican who is close to the president.
“Trump understands the dynamic of the top two, and the need to focus on one candidate if Republicans have any hope to make it to November,” Kousser said. “And Cox has been ahead."
Cox, a businessman from San Diego, has actively courted Trump supporters in the primary, despite voting for Libertarian candidate Gary Johnson in the 2016 presidential election. At a debate earlier this month, Cox said he regrets that decision.
"I wasn't sure he's a conservative," he said of Trump. "I am now. He's a conservative."
The latest polling from the Berkeley Institute of Governmental Studies found Cox with 18 percent of the vote, just ahead of GOP Assemblyman Travis Allen, who polled at 16 percent.
Cox said Friday that he believes his background as a businessman is what led Trump to back him over Allen.
"I think the fact that I'm a businessman," Cox added. "Like him I focus on results. I focus on getting solutions to problems."
Kousser said that Democratic front-runner Newsom’s campaign is likely celebrating the endorsement as well.
At a recent debate, Newsom was clear that he’d prefer to face either Cox or Travis Allen in November.
“I think they are probably toasting with beer in San Diego, and toasting with chardonnay in San Francisco,” Kousser added, referring to the hometowns of Cox and Newsom, respectively. “To his credit, Gavin Newsom has been upfront about his hoping to face a Republican in November — he knows he can beat every Republican, but any Democrat is a threat to him.”
Newsom took to Twitter to respond to Friday's endorsement.