The 28-year old Japan international, who joined the Saints from Sanfrecce Hiroshima in January 2012, has been unable to force his way into Mauricio Pochettino's plans and has subsequently departed St Mary's.
"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," read a club statement.
"A couple of injury setbacks, however, denied the Japanese forward from breaking into the team on a regular basis.
"Southampton thanks Lee for his services and wishes the forward well in his future career."
During his time on the south coast, Lee made seven appearances in the Championship for one goal in the promotion-winning 2011/12 season as well as seven cups outings for one more goal.
Last season, he enjoyed a loan spell with FC Tokyo where he scored six goals in 19 appearances in all competitions.
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