DeGeneres had conducted a "George Watch '08" campaign to get Clooney. Her theme this year was "George Clooney Is Mine In '09."
Clooney's "ER" co-star Noah Wyle told DeGeneres that some Big Red Machine players from the 1970s might pique Clooney's interest because he grew up around Cincinnati and is a big Reds fan.
Team executives helped line up the appearance by the baseball players.
The show was taped Thursday in Burbank, Calif., and will be broadcast Monday.