The 25-year-old, who played for Skoda Xanthi, lost control of his car on the Egnatia motorway before the crash.
Adefemi was driving from Xanthi to Salonika in the Kavala region of northern Greece.
The former Rapid Bucharest player was on his way to Nigeria to finalise details of his wedding when the accident happened, police said.
"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.
Adefemi's Zambian team-mate and friend Chris Katongo says Xanthi players were in shock when they heard news of the death of the popular player.
"It was a good day and all the players were laughing and joking when the manager came in and broke the news. Everyone is in shock," Katongo told BBC Sport.
"Even as I am speaking I am thinking it is a joke but it is true. I can't believe this.
"He was a good person, he likes to joke and likes to laugh. It is the reason why he got along with everyone and was well liked.
"He was like a brother to me because we shared the same room during team camping. It is a big loss to me and the entire Xanthi team."
Adefemi's car skidded off the highway in Kavala in bad weather conditions before crashing into a barrier, killing him instantly.
"For the past three of four days in that part of Greece the weather has been terrible and unfamiliar with the country," Greek sports journalist Christos Sotirakopoulos told BBC Sport.
"The road has been slippery with heavy rains and in some cases heavy snow which is quite unusual for Greece."
Meanwhile, Skoda Xanthi will organise a friendly match in the memory of the Nigerian defender to raise fund to support his family.
"His team have already said that they will organise a game in his memory to send some money to his family in Africa," Sotirakopoulos
Adefemi signed for Xanthi last summer and made 24 appearances for the club, scoring twice.
He has also played five times for Nigeria.
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