The tweet, originally written in Greek, references the appearance of Nile virus-carrying mosquitoes in Athens: "With so many Africans in Greece… At least the West Nile mosquitoes will eat home made food!!!"
The triple-jumper previously caused controversy by re-tweeting a comment criticising immigration from Ilias Kasidiaris, a politician part of the far-right Golden Dawn party, famous for slapping a female politician live on television.
The Greek Olympic Committee released a statement yesterday (July 25) confirming Papachristou's expulsion from the team.
Papachristou also made a statement on Twitter yesterday, apologising for the racist tweet: "I am very sorry and ashamed for the negative responses I triggered, since I never wanted to offend anyone, or to encroach human rights.
"I would like to apologise to all my friends and fellow athletes, who I may have insulted or shamed, the National Team, as well as the people and companies who support my athletic career. Finally, I would like to apologise to my coach and my family."
Photo gallery - London 2012 Olympics: