The 24-year-old, who only returned to action in January after a long-term injury, said on his Facebook page: "The start of a new adventure. I'm happy to return to Inter. Thanks to Newcastle... the city will remain in my heart."
Following an eight-month lay-off with an ongoing knee problem, Santon played his first game of the season on January 3 in Newcastle's 1-0 FA Cup defeat at Leicester.
There has been previous interest from Serie A side Inter Milan, where he spent three years from 2008 and was given his debut by manager Jose Mourinho.
But it is a surprise Santon has opted for a switch back to his homeland following quotes from the Italian on January 14.
The full-back said then: "I am happy here - I still have one and a half years left on my contract and I will probably have to speak to Newcastle soon. I’ve always said I am happy here."
After a four-month loan spell with Cesena in 2011, Santon was brought to St James' Park by Alan Pardew for £5million.
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NEWCASTLE UTD v STOKE CITY
February 7th
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Newcastle Utd Fixtures ...
2014/15 SEASON
Sunday August 17
NEWCASTLE 0 MAN CITY 2
Krul, Janmaat, Williamson, Coloccini, Dummett, Colback, Anita (63' Obertan), Cabella, Sissoko, Gouffran (74' Aarons), Riviere (83' Perez).
Subs: Haidara, Elliot, S Taylor, Abeid
Saturday August 23
ASTON VILLA 0 NEWCASTLE 0
Krul, Janmaat, Williamson, Coloccini, Dummett, Anita (70' de Jong), Colback, Cabella, Sissoko, Gouffran, Riviere (77' Perez - 90+2' S Taylor).
Subs: Haidara, Elliot, Obertan, Abeid
Tuesday August 26
LC GILLINGHAM 0 NEWCASTLE 1 (Egan OG)
Krul, Janmaat, Coloccini, S Taylor, Haidara, Sissoko, Abeid, Obertan, de Jong (68' Perez), Aarons (60' Cabella), Riviere (86' Gouffran)
Subs: Anita, Ferreyra, Elliot, Dummett
Saturday August 30
NEWCASTLE 3 CRYSTAL PALACE 3 (Janmaat, Aarons, Williamson)
Krul, Janmaat (79' Perez), Williamson, Coloccini, Haidara, Sissoko, Colback, Cabella, de Jong (79' Anita), Gouffran (67' Aarons), Riviere
Subs: Elliot, Obertan, S Taylor, Dummett
Saturday September 13
SOUTHAMPTON 4 NEWCASTLE 0
Krul, Janmaat, Williamson, Coloccini, Haidara, Anita (45' Tiote), Colback, Cabella (69' Perez), Sissoko (90'Ameobi), Gouffran, Riviere
Subs: Elliot, Obertan, S Taylor, Armstrong
Saturday September 20
NEWCASTLE 2 HULL CITY (Cisse 2)
Krul, Janmaat, Williamson, Coloccini, Dummett, Tiote (85' Ameobi), Colback, Sissoko, Cabella, Gouffran (90+4' Perez), Riviere (69' Cisse)
Subs: Anita, Haidara, Elliot, S Taylor
Wednesday September 24
LC CRYSTAL PALACE 2 NEWCASTLE 3 AET (Riviere 2, Dummett)
Elliot, Janmaat (46 Haidara), Coloccini, S Taylor, Dummett, Colback, Abeid, Obertan, Armstrong, Ameobi (66 Sissoko), Riviere (80 Perez)
Subs: Anita, Ferreyra, Cabella, Alnwick
Monday September 29
STOKE CITY 1 NEWCASTLE 0
Krul, Janmaat, Williamson, Coloccini, Dummett, Tiote, Colback, Sissoko, Cabella (74' Ameobi), Gouffran (68' Obertan), Riviere (45' Cisse)
Subs: Anita, Elliot, S Taylor, Armstrong
Saturday October 4
SWANSEA CITY 2 NEWCASTLE 2 (Cisse 2)
Krul, Janmaat, Williamson, Coloccini, Dummett, Obertan, Tiote, Sissoko, Colback (69' Riviere), Gouffran (58' Ameobi), Cisse (85' Perez).
Subs: Anita, Haidara, Cabella, Elliot
Saturday October 18
NEWCASTLE 1 LEICESTER CITY 0 (Obertan)
Krul, Janmaat, S Taylor, Coloccini, Dummett (72' Haidara), Tiote, Anita (59' Perez), Colback, Obertan, Sissoko, Gouffran (90+2' Cabella), Cisse
Subs: Elliot, Abeid, Armstrong
Saturday October 26
TOTTENHAM 1 NEWCASTLE 2 (Ameobi, Perez)
Krul, Janmaat, S Taylor, Coloccini, Dummett, Anita (45' Cabella), Colback, Obertan (45' Ameobi), Sissoko, Gouffran, Perez (79' Haidara)
Subs: R Taylor, Ferreyra, Elliot, Armstrong
Wednesday October 29
LC MAN CITY 0 NEWCASTLE 2 (Aarons, Sissoko)
Elliot, Janmaat (64' Sissoko), Coloccini, Dummett, Haidara, Colback, R Taylor, Abeid, Obertan, Armstrong (65' Riviere), Aarons (46' Ameobi)
Subs: Krul, Gouffran, Perez, Cabella
Saturday November 1
NEWCASTLE 1 LIVERPOOL 0 (Perez)
Elliot, Krul, Janmaat, S Taylor, Coloccini, Dummett, Abeid, Colback, Obertan (27' Aarons), Sissoko), Ameobi (66' Cabella), Cisse (45' Perez)
Subs: R Taylor, Gouffran, Haidara, Elliot
Sunday November 9
WEST BROM 0 NEWCASTLE 2 (Perez, Coloccini)
Krul, Janmaat, Coloccini (78' Williamson), Dummett, Haidara, Abeid, Colback, Cabella (82' R Taylor), Sissoko, Ameobi (78' Gouffran), Perez
Subs: Anita, Cisse, Elliot, Armstrong
Saturday November 22
NEWCASTLE 1 QPR 0 (sissoko)
Krul, Janmaat, Williamson, Dummett, Haidara, R Taylor (34' Gouffran), Colback, Cabella (68' Cisse), Sissoko, Ameobi, Perez (89' Armstrong)
Subs: Anita, Ferreyra, Elliot, Streete
Saturday November 29
WEST HAM 1 NEWCASTLE 0
Elliot, Janmaat (74' S Taylor), Williamson, Dummett, Haidara, Tiote (63' Cabella), Colback, Gouffran (63' Cisse), Sissoko, Ameobi, Perez
Subs: Anita, Riviere, Alnwick, Armstrong
Tuesday December 2
BURNLEY 1 NEWCASTLE 1 (Cisse)
Elliot, Janmaat, Williamson (45' S Taylor), Dummett, Haidara, Abeid, Tiote, Gouffran, Perez (45' Cabella), Ameobi (75' Riviere), Cisse
Subs: Anita, Alnwick, Armstrong, Streete
Saturday December 6
NEWCASTLE 2 CHELSEA 1 (Cisse 2)
Elliot (45' Alnwick), Janmaat, S Taylor, Coloccini, Dummett, Tiote, Colback, Cabella (53' Cisse), Sissoko, Ameobi (83' Williamson), Perez
Subs: Anita, Gouffran, Haidara, Riviere
Saturday December 13
ARSENAL 4 NEWCASTLE 1 (Perez)
Alnwick, Janmaat, Williamson, Coloccini, Dummett, Tiote, Colback, Perez (82' Armstrong), Ameobi (56' Cabella), Gouffran, Cisse (56' Riviere)
Subs: Anita, Haidara, Vuckic, Woodman
Wednesday December 17
LC TOTTENHAM 4 NEWCASTLE 0
Alnwick, Coloccini, Williamson, Haidara, Dummett, Colback, Cabella (80' Vuckic), Gouffran, Perez, Sissoko (73' Anita), Riviere (60' Armstrong)
Subs: Streete, Satka, Woodman, Roberts
Sunday December 21
NEWCASTLE 0 SUNDERLAND 1
Alnwick, Janmaat, S Taylor, Coloccini, Dummett, Tiote (75' Cisse), Colback, Gouffran (59' Armstrong), Sissoko, Ameobi (89' Cabella), Perez
Subs: Williamson, Haidara, Riviere, Woodman
Friday December 26
MANCHESTER UNITED 3 NEWCASTLE 1 (Cisse pen)
Alnwick, Janmaat, S Taylor, Coloccini, Dummett (63' Cabella), Anita, Colback, Gouffran, Sissoko, Armstrong (63' Cisse), Perez (82' Vuckic)
Subs: Williamson, Riviere, Satka, Woodman
Sunday December 28
NEWCASTLE 3 EVERTON 2 (Cisse, Perez, Colback)
Alnwick, Janmaat, Williamson, Coloccini, Dummett, Tiote, Colback, Sissoko, Perez, Gouffran (90+1' Cabella), Cisse (73' Riviere)
Subs: Santon, Anita, S Taylor, Armstrong, Woodman
Thursday January 1 2015
NEWCASTLE 3 BURNLEY 3 (S Taylor, Sissoko, Colback)
Alnwick, Janmaat, Coloccini, S Taylor (Williamson 61), Dummett, Sissoko, Colback Tiote, Gouffran (Haidara 71), Perez, Riviere (Armstrong 57)
Subs: Santon, Anita, Satka, Woodman
Saturday January 3 2015
FAC LEICESTER CITY 1 NEWCASTLE 0
Alnwick, Santon, Williamson, Dummett, Haidara, Tiote, Anita, Vuckic (79' Roberts), Armstrong, Riviere, Cabella
Subs: Gouffran, Campbell, Satka, Bigirimana, Woodman, Gilliead
Saturday January 10 2015
CHELSEA 2 NEWCASTLE 0
Krul, Janmaat, Williamson, Coloccini, Dummett, Anita, Colback, Cabella, Sissoko (84' Riviere), Gouffran (63' Ameobi), Perez
Subs: Santon, Haidara, Vuckic, Satka, Woodman
Saturday January 17 2015
NEWCASTLE 1 SOUTHAMPTON 2 (Gouffran)
Krul, Janmaat, Coloccini, Dummett (80' Williamson), Haidara, Anita (68' Ameobi), Colback, Cabella, Sissoko, Gouffran (68' Riviere), Perez
Subs: Santon, Abeid, Armstrong, Woodman
Saturday January 31 2015
HULL CITY 0 NEWCASTLE 3 (Cabella, Ameobi, Gouffran)
Krul, Janmaat, Williamson, Coloccini, Haidara, Anita, Colback, Cabella (72' Gouffran), Sissoko, Ameobi (90+2' Abeid), Perez (82' Cisse)
Subs: Santon, Obertan, Riviere, Alnwick
Saturday February 7 2015
Newcastle United v Stoke City
Tuesday February 10 2015
Crystal Palace v Newcastle United
Saturday February 21 2015
Manchester City v Newcastle United
Saturday February 28 2015
Newcastle United v Aston Villa
Wednesday March 4 2015
Newcastle United v Man Utd
Saturday March 14 2015
Everton v Newcastle United
Saturday March 21 2015
Newcastle United v Arsenal
Saturday April 4 2015
Sunderland v Newcastle United
Saturday April 11 2015
Liverpool v Newcastle United
Saturday April 18 2015
Newcastle United v Tottenham
Saturday April 25 2015
Newcastle United v Swansea City
Saturday May 2 2015
Leicester City v Newcastle United
Saturday May 9 2015
Newcastle United v West Brom
Saturday May 16 2015
QPR v Newcastle United
Sunday May 24 2015
Newcastle United v West Ham United