The minibus carrying 10 men was travelling from Greece to France when it collided with a truck in Timis county in western Romania, local police said.
Greek Prime Minister Kyriakos Mitsotakis said he was "deeply shaken" when he "learned of the tragic accident in Romania that claimed the lives of seven young compatriots".
"In these difficult moments, together with all Greeks, I extend my most sincere condolences to the victims' loved ones and to the PAOK family," he added on Facebook, referring to the PAOK football club the men supported.
A 29-year-old man was driving the minibus "when he crossed into the opposite lane and collided head-on with a truck", Timis police said in a press release.
Of the seven who died, six victims were pronounced dead at the scene, Romanian emergency services told AFP, while three others were hospitalised with injuries.
The minibus was transporting PAOK Thessaloniki supporters to an away game, according to Thessaloniki mayor Stelios Angeloudis.
PAOK are set to play against Lyon on Thursday in the Europa League.