The 28-year-old joined the south coast club from Sanfrecce Hiroshima in January 2012 and made seven appearances in Southampton's promotion-winning Championship campaign.
However, Lee, who won Southampton's goal of the season competition with his first strike for the club, has failed to break into the first team since the club's promotion to the top flight, finding opportunities limited under Mauricio Pochettino, who replaced former boss Nigel Adkins in January 2013.
After making 19 appearances on loan at FC Tokyo in his homeland last year, Lee has now been allowed to leave St Mary's Stadium.
A statement on the club's website read: "Lee played a vital role in the team’s promotion run with valuable assists in the games against Barnsley and Leeds United, which saw him win a place in Saints’ Premier League squad on the club’s return to the top-flight."
"A couple of injury setbacks, however, denied the Japanese forward from breaking into the team on a regular basis.
"Southampton Football Club thanks Lee for his services and wishes the forward well in his future career."
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