Turkey president Recep Tayyip Erdogan has given his support to Mesut Ozil and says the "racist attitude" shown towards the player is not acceptable.
The 29-year-old Arsenal midfielder was criticised after being photographed with Erdogan at a London event in May.
Ozil quit the Germany national team, saying he has faced "racism and disrespect" over his Turkish roots.
The Turkish Football Federation (TFF) also offered its "full support to Mesut Ozil and his family".
The TFF said in a statement: "We condemn the treatment, threats and derogatory messages that Ozil has received because of his heritage and background.
"Every player, no matter whether they are in the public eye or not, has a right to be protected from abuse, discrimination and messages expressing hate."
Erdogan added: "On Monday night I spoke to Mesut. His attitude in the statement is completely patriotic.
"It is not possible to accept this kind of racist attitude towards a young man who poured so much sweat for the German national team's success. This cannot be tolerated."
In a lengthy statement released on Monday, Ozil said he received hate mail and threats and was being blamed for Germany's disappointing group-stage exit at the World Cup.
The third-generation Turkish-German was born in Gelsenkirchen and was a key member of his country's 2014 World Cup-winning side.
He has 92 caps and has been voted the national team's player of the year five times since 2011 by fans.
"I am German when we win, but I am an immigrant when we lose," Ozil said. "I used to wear the German shirt with such pride and excitement, but now I don't."