The games will be played March 28-29 at the Tokyo Dome, marking the fourth season-opening trip to Japan for Major League Baseball.
"Major League Baseball is very pleased to give Japanese fans a chance to see the Seattle Mariners and the Oakland Athletics, two franchises with great popularity in Japan," said Baseball Commissioner Bud Selig in a statement. "Opening Series 2012 will be an excellent opportunity to bring the game of baseball to fans across the globe and to celebrate the many contributions to the game by Japanese players. Most importantly, with the shared love of baseball between our nations, I believe that we can use this event to further assist the ongoing relief efforts throughout Japan."
In addition, both teams will play exhibition games against Japanese teams.
Oakland is scheduled to be the home team for both games. The Mariners and Athletics will return to the United States to complete exhibition play, then resume the regular season slate in April.
To account for the two regular season games that will be played in Tokyo, the previously announced games at Oakland on Sunday, April 8, and Thursday, July 5, will now serve as off days for both clubs.
The A's and Mariners were scheduled to open the 2003 season in Japan, but the trip was canceled the day after war broke out in Iraq.