International Olympic Committee (IOC) president Thomas Bach delivered the ball to be used in the ceremonial first pitch.
The honours went to two Japanese students from areas affected by the 2011 Japan earthquake and tsunami.
Dominican Republic takes on Japan in the opening game at the Fukushima Azuma Stadium.
Baseball was played as a demonstration at a number of Games before being adopted as a medal sport at Barcelona in 1992.
It was part of the Games until Beijing in 2008 and then removed from the programme.
At Tokyo, teams representing six nations will compete in the sport, including the U.S, Mexico, South Korea, and Israel.