The hotly contested race in Georgia, where Democrat Stacey Abrams was seeking to become the first black woman to be elected governor of a US state, remained too close to call early on Wednesday.
The race was among the most watched in the USA midterms, with Trump throwing his weight behind DeSantis and Gillum endorsed by former Democratic president Barack Obama and former presidential candidate Bernie Sanders.
DeSantis' victory as the next governor of Florida will also have a lasting impact on the state's Supreme Court.
Gov. Rick Scott snatched the seat for the GOP from Sen. Gillum, a liberal Democrat, and DeSantis, who has tried to imitate Trump in every way, provided voters with a pretty intense contrast.
In recent days, developers Don Peebles and Jeff Greene each gave $100,000 to Forward Florida, a political action committee supporting Gillum.
Part of the reason Gillum, who consistently led in polls, lost is that only two weeks before the midterm election, news broke that he had accepted tickets to see the Broadway play Hamilton from an undercover Federal Bureau of Investigation agent posing as a real-estate developer.
DeSantis won with 49.7 percent of the votes, with ballots in his favour totalling 4,052,118 votes.
Gillum conceded victory to DeSantis Tuesday night after narrowly trailing behind.
"So is it a red state?"
"We'll worry about history later but today, we're working to win".
"We didn't shrink from the challenges, we didn't shrink from power", Gillum added, according to TampaBay.com.
As governor, DeSantis will essentially be Trump lite.
Republicans controlled 33 governors' mansions and two-thirds of state legislative chambers going into Tuesday.
Trump has dubbed the Republican results in the House and Senate a "success".
The president injected himself into the race in December when, while taxiing on Air Force One on the runway at Palm Beach International Airport, he tweeted that DeSantis "would make a GREAT Governor of Florida".
During the campaign, Gillum repeatedly launched accusations of racism against DeSantis, which DeSantis firmly denied.
Trump was heavily invested in the race, making two visits to the state in the final week of the campaign.
In Colorado, Democrat Jared Polis was set to become the first openly gay man to be elected governor of a U.S. state, after NBC News projected him to win against Republican Walker Stapleton.
Under Georgia law, if neither Abrams nor Kemp wins a majority of the vote on Tuesday, their battle continues to a December runoff.