Santon confirmed the switch earlier on Monday when he took to Facebook to state he was returning to San Siro.
The full-back departed San Siro for Premier League club Newcastle in August 2011 and has largely been a first-team regular at St James' Park.
However, a knee injury has plagued Santon this term and he has failed to make a single league appearance in 2014-15.
"Internazionale is pleased to announce the return on Davide Santon," read a statement on the club's official website. "Santon joins on loan with the right to purchase."
Elsewhere at Inter, teenage forward Federico Bonazzoli has been sold to fellow Serie A outfit Sampdoria.
The 17-year-old will remain on loan with Roberto Mancini's side for the rest of the campaign.
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