The former presidential hopeful defeated several GOP challengers vying for his seat.
Paul, a first-term senator, successfully lobbied
last year for the Kentucky Republican Party to change its May presidential primary to a caucus in March.
Kentucky law forbids a single candidate from appearing on the same ballot more than once. The Kentucky GOP's move allowed Paul to sidestep this rule.
Paul will face off against Democratic nominee Jim Gray, the mayor of Lexington, in the November general election, according to a CNN projection. According to his campaign website, Gray is the first openly gay mayor of Lexington. If elected, Gray would be the first openly gay man in the U.S. Senate.
Paul, the son of former Texas politician and libertarian-leaning stalwart Ron Paul, dropped out
of the presidential contest after the Iowa caucuses in February.
Paul was an outspoken critic of Republican presumptive nominee Donald Trump, and since ending his bid, has largely stayed away from weighing in on the fractious GOP primary fight.
Kentucky has been a solidly Republican state in recent cycles, but still hosted a contentious Senate race
in 2014 for the state's senior senator, GOP Majority Leader Mitch McConnell.