The last debate before the key South Carolina primaries later this month was the most watched (13.51 million viewers) so far this election year, the network that hosted Saturday's clash said, citing Nielsen.
The event hit its peak audience between 10:00 and 10:30 pm (0100 and 0130 GMT Sunday) with 14.63 million viewers, CBS added.
It was also this year's highest-rated debate in adults aged 25-54.
Held in Greenville, South Carolina, the debate quickly turned nasty and divisive, with billionaire Donald Trump, former Florida governor Jeb Bush and Senator Ted Cruz exchanging heated verbal blows.
It was an extraordinary back and forth on the national stage. Candidates often ignored the moderators and went after each other in perhaps the most aggressive exchanges of the nine Republican debates to date.
The broad Republican field has narrowed to six candidates, led by Trump.