DeSantis, 40, a conservative former congressman, defeated Democrat Andrew Gillum, 39, the staunchly liberal mayor of state capital Tallahassee who was seeking to become Florida’s first black governor.
Mr Gillum has conceded the race.
Florida is the most densely populated of the battleground states that swing between the parties in presidential elections, but Republicans have controlled the governor’s mansion for two decades.
Mayor Andrew Gillum of Tallahassee, running as a liberal candidate, successfully pushed to become the state’s first Democratic governor in two decades, and he opened a narrow lead in many polls over the Republican candidate, Ron DeSantis, a former congressman.
Mr DeSantis has allied himself with President Trump and attempted to embroil Mr Gillum in a continuing FBI investigation into possible corruption at the Tallahassee City Hall.
But he stumbled early when he warned electing Mr Gillum would “monkey this up”.
On issues like climate change, which is increasingly important in Florida, Mr DeSantis has said he didn’t want to be known as an “alarmist,” while Mr Gillum promised to be a governor who “believes in science”.
The Senate race has also been closely watched, with the state’s wealthy Republican governor, Rick Scott, challenging the three-term incumbent, Senator Bill Nelson, a low-key Democrat who flew on the space shuttle in his former career as an astronaut.
The city of Miami was seen as a key House battleground in Florida.
In the 26th District, Carlos Curbelo has run well to his party’s left on issues like climate change and immigration, and Republicans need him to survive.