The 27-year-old attacking midfielder, who struck eleven goals in 80 games at Reggina following a move from Yokohama F-Marinos in 2002, has won 52 caps for Japan and scored in one of three FIFA Confederations Cup appearances this summer.
"He starred against Brazil and was man of the match," said Celtic manager Gordon Strachan. "I have done a lot of homework and have spoken to people who have worked with him all over the place, so I know what he's like. He's great going forward, with his vision, and he sees things a lot of people can't see. He can run with the ball and he is a decent lad as well."
Celtic take on FC Artmedia Bratislava in the first leg of their UEFA Champions League second qualifying round tie on Wednesday, although Nakamura - who has signed a three-year contract - will not be eligible for the competition until the next round, as the registration deadline passed last Thursday.
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