The 20-year-old headed for England after the two clubs agreed a fee, understood to be in the region of £5million, with the Magpies' search for a replacement for Jose Enrique nearing a conclusion.
Santon, who was described as "the new Paolo Maldini" earlier in his career, spent the second half of last season on loan at Cesena after falling out of favour at Inter, initially under former manager Jose Mourinho.