The 25-year-old defender lost control of his car on the Egnatia motorway while driving from Xanthi to Salonika in the Kavala region of Northern Greece.
The former Rapid Bucharest player was on his way to finalise details of his wedding when the accident happened, police said.
Adefemi signed for Xanthi last summer and made 24 appearances for the club, scoring twice. He has also played three times for Nigeria.
"The Super League and the entire Greek football family would like to express its deepest condolences and sympathy for the family of Olubayo Adefemi who died tragically in a car accident," league organisers said in a statement on Monday.
May his soul rest in perfect peace.
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