The 29-year-old has appeared 93 times for Japan, netting 43 times in the process.
He has scored 27 times in the Bundesliga over the last two seasons, and joins Robert Huth and Christian Fuchs as the new signing at the King Power ahead of the start of next season.
"I'm very happy to be here. To play in the Premier League is a dream," Okazaki told the Leicester's official website.
"Last season the crowd were amazing and the players responded. I hope to show everyone what I can do.
"I'm excited to find out what I can do in this league. I will try to make lots of goals for the supporters."