The Samurai Blue right-back will join the Bundesliga outfit this summer on a three-year contract for a reported €1 million transfer fee after terms were reached this week
The 22-year-old right-back will join on a three-year contract, for which Reysol will receive an estimated €1 million in transfer fees.
Expected to participate in the London Olympics, Sakai made his top-team debut for Japan in a May friendly against Azerbaijan before coming on as a substitute in his team's 3-0 victory against Oman on Sunday night in the final round of World Cup qualifying.
"Out of all the teams that made offers, [Hannover] has been interested since last winter so I feel comfortable with them," Sakai told reporters after watching Friday night's 6-0 win over Jordan from the bench.
"It's a new start for me, but even when I go overseas I'll still play hard to represent Reysol."
The player will travel with Japan to Brisbane, where the current Group B leaders will play Australia on Tuesday night. The transfer is expected to be officially announced next week, with a press conference scheduled after the J-League resumes play on June 16.