The 35-year-old was widely regarded as the "water carrier" for the more creative Hidetoshi Nakata in an international career that spanned 67 games since his Japan debut in 1995.
Nanami became the third Japanese player after Kazuyoshi Miura and Nakata to play in Italy's Serie A when he left J-League Jubilo Iwata to join Venezia in 1999.
Iwata officials said Nanami would formally announce his retirement at a news conference on Friday.
Nanami played alongside Nakata for Japan in the country's first World Cup finals appearance in France in 1998, though the team lost all three group matches.
He was also voted player of the tournament when Japan won their second Asia Cup title in Lebanon in 2000.
Following his return after a frustrating year in Italy, Nanami rejoined Iwata and also had spells at Cerezo Osaka and Tokyo Verdy before ending his career at Jubilo.
