The program, produced by CNN and presented by two of its best-known journalists, fell 35 percent short of the 73.1 million viewers who tuned in on September 29, 2020, which pitted the same two candidates against each other for the first time.
The second debate of the 2020 presidential election attracted 63 million viewers.
Despite this marked decline, the audience for Thursday's live broadcast was the largest seen this year in the US outside of sporting events, CNN said in a statement Friday.
Viewer ratings agency Nielsen put the audience a notch higher than CNN's count, at 51.3 million viewers across the US networks.
As host of the debate, CNN offered to broadcast it on other channels, which all major American stations agreed to do, including its rival, Fox News.
Just over nine million viewers tuned in on CNN, ahead of Fox News (8.8 million), according to the channel's own figures.
CNN clocked up 11.4 million viewers across all of its networks, 27 percent higher than the number who tuned in for the first debate of 2020.
The debate marked what may yet prove a turning point for Biden, who repeatedly hesitated, tripped over words and lost his train of thought -- a performance that contrasted with Trump's usual pugnacity and false or misleading statements, most of which went unchallenged.
The American media reported a wave of concern in the Democratic camp after the debate, with many pundits calling for the president to throw in the towel. Biden dismissed this scenario on Friday.
