The 27-year-old Italian, who joined Saints in January, 2017 from Napoli for a reported 14 million pounds, has returned to seventh-placed Samp, where he played from 2013-15, to help the Serie A side qualify for Europe.
Gabbiadini scored 12 goals in 60 games for Southampton, including two in the 3-2 League Cup final defeat by Manchester United in 2017. He also got the winner at Swansea City last season, which effectively ensured Saints' top-flight status.
However, he has only scored once in 15 games this season.
Southampton's Austrian manager Ralph Hasenhuettl said the Italian did not fit in with his preferred pressing style.
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"I think with this intensive pressing system we try to play, it is not his way, so it was better to find a solution which is better for both sides," he was quoted as saying by the BBC.
"It is a good step for him to go back to Italy, I wish him all the best. I'm sure he can score goals at every club in the world, but we try to go another way and for that we need other players."
(Writing by Ken Ferris; Editing by Ed Osmond)
