About 300 fans greeted the teams after they landed at Tokyo's Narita Airport, with Mariners outfielder Ichiro Suzuki drawing most of the attention as the team made its way through the terminal.
Utility infielder Munenori Kawasaki and pitcher Hisashi Iwakuma are the other Japanese players here for Seattle.
Suzuki hit his second home run of the spring Wednesday against the Chicago White Sox. The Seattle outfielder is hitting .400 since moving to the third spot in the lineup this spring after batting leadoff for nearly all of his career.
The teams open the regular season with a two-game series on Wednesday and Thursday.
"We're thrilled to be back in Japan," Oakland general manager Billy Beane said. "When they asked who wanted to go we were the first to put up our hands."
Seattle and Oakland had been scheduled to play here in March 2003, but the series was scrapped because of the threat of war in Iraq.
The A's will be the home team in both games.
Beane said he expects Suzuki will be just as tough to get out in Japan as he is in the United States.
"Suzuki is tough on us back home and it won't be any easier here," Beane said.
The teams left their spring training homes in Arizona on Thursday and will return to Arizona in a week. They will play preseason games against the Yomiuri Giants and Hanshin Tigers on Sunday and Monday.
"It was a long flight but everyone feels pretty good," Seattle infielder Chone Figgins said. "This is my third time here and it's always nice to be back in Japan."
Oakland's Manny Ramirez did not make the trip. Ramirez must sit out the first 50 games for a second violation of MLB's drug policy, making him eligible to play his first game barring rainouts on May 30 -- his 40th birthday.
Major League Baseball and the players' association said the series also will aim to assist rebuilding in Japan following last year's earthquake. On Tuesday, a group of players will travel to Ishinomaki in the disaster-hit northeast region to conduct a baseball clinic.