The fee for the 29-year-old, who is prolific in front of goal, is undisclosed but German media reports said it was just over 10 million euros (£7.1 million).
"I hear the Premier League is a very physical division and better for individual talent so I’m excited to know what I can do in this league," Okazaki told Leicester's website (www.lcfc.com) on Friday.
He will be the club's third signing of the transfer window following German defender Robert Huth from Stoke City and Austria left back Christian Fuchs from Schalke 04.
"Regarded as one of the finest forwards in the German league, Okazaki brings with him an exciting and explosive playing style alongside an impressive international pedigree," said Leicester.
His 43 goals in 93 internationals make him Japan’s third top scorer while 12 league goals for Mainz last term saw him become the Bundesliga's all-time leading Japanese marksman.
The 5-foot-8 (1.72 metres) Okazaki is known for spectacular goals and, surprisingly for a small man, gets many with his head but also endears himself to fans with tireless workrate.
After playing for the under-23s at the 2008 Beijing Olympics, Okazaki established himself as the senior side's first-choice striker thanks to some impressive performances.
He scored his first goal for Japan in 2009 against Yemen, the first of 15 that year including back-to-back hat-tricks against Hong Kong and Togo.
Another treble in the 2011 Asian Cup campaign came in between the 2010 and 2014 World Cups during which Okazaki played and scored.
He started out with J-League side Shimizu S-Pulse, signing his first professional contract in 2005. After making his debut aged 19 he scored 49 goals in 154 games before joining VfB Stuttgart in January 2011.
Okazaki moved to Mainz in 2013, netting on his debut during a 3-2 victory over former club Stuttgart, and finished with 15 goals to help them qualify for the Europa League.
When he makes his debut for Leicester he could become the seventh Japanese player to feature in the Premier League.
($1 = 0.8964 euros)
(Writing by Ken Ferris, editing by Tony Jimenez)
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