"My heart goes out to the people of Japan during this enormous tragedy. Please know that America will always stand by one of its greatest allies during their time of need," Obama wrote in the book.
"Because of the strength and wisdom of its people, we know that Japan will recover and indeed will emerge stronger than ever. And as it recovers, the memory of those who have been lost will remain in our hearts, and will serve only to strengthen the friendship between our two countries. May God Bless the people of Japan."
Japanese Ambassador Ichuro Fujisaki thanked Obama and the American people, saying "We are working 24 hours, and his assistance means a lot to all Japanese."
ABC News' Sunlen Miller, Luis Martinez and Akiko Fujita contributed to this report.