Final Nielsen ratings data on Tuesday showed that 59.2 million Americans watched Monday's third match-up between the two men before the November 6 presidential elections.
That figure was 8 million below the 67.2 million who tuned in for their first encounter on October 3. Some 65.6 million viewers watched the second debate last week.
Monday's foreign policy debate was carried by 11 cable and broadcast networks. But Romney and Obama were competing against the Major League Baseball playoffs on Fox television - seen by 8.1 million - and ESPN's Monday Night Football, which drew an audience of 10.7 million, Nielsen said.
None of the presidential debates this year have captured Americans' attention like the 2008 vice-presidential encounter between Republican Sarah Palin and Democrat Joe Biden.
It was watched by 69.9 million people and ties as the second most-viewed debate ever.
The most-watched presidential debate on U.S. television was the October 1980 encounter between Ronald Reagan and Jimmy Carter. It drew 80.6 million viewers, according to Nielsen, which began collecting debate data in 1976.
