The incumbent senator Raphael Warnock defeated Republican Herschel Walker, a former football star and protege of former president Donald Trump.
Joe Biden's party has won a new US Senate seat with Democrat Raphael Warnock's victory in Georgia, according to television networks.
Tuesday's victory confirms the very slim Democratic majority – 51 to 49 – in the upper house of Congress.
The incumbent senator defeated Republican Herschel Walker, a former football star and protege of former president Donald Trump.
Even before CNN and NBC called the race for Warnock, Biden appeared confident, shouting at a distance to pool reporters as he arrived in Washington: "We're going to win. We're going to win Georgia."
Warnock's victory does not change the balance of power in the Senate, which Democrats had already secured control of on November 8.
But the victory allows Democrats to wield greater control in committees and curbs the power of any individual Democratic senator to sink Biden initiatives.
The Republicans took back the House, but with a much smaller majority than expected.
Warnock, pastor at the Atlanta church where civil rights icon Martin Luther King Jr once preached, and Walker, who are both African American, faced voters after neither earned more than 50 percent in the November 8 midterm vote.
With Warnock, 53, and Walker, 60, running neck and neck, Biden urged Georgians on Tuesday to turn out and vote.
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"Georgia, today is Election Day – and the eyes of the nation are on you. Head to the polls and help send @ReverendWarnock back to the US Senate," the president tweeted.
Democrats retained control of the Senate in last month's vote – but just barely, winning 50 seats.
Vice President Kamala Harris's tie-breaking vote gives Democrats the edge in the 100-seat chamber.
A Warnock win gives Democrats 51 seats and significantly curbs the power of centrist Democratic senator Joe Manchin, who has already blocked several major Biden initiatives in the first two years of his term.
With 700 days to go before the 2024 presidential election, Republicans hope to stymie Biden's momentum, after his party performed much better than expected in November.