The right-wing Orban has angered and infuriated his European Union peers for years, carrying out sweeping constitutional and institutional changes that critics say have curbed press freedom and judicial authority.
"Hello, dictator," Juncker was overheard to say to Orban in front of the press at the EU-Eastern Partnership summit in Latvia.
Reporters said they had not been able to hear what Orban had said in reply, if anything, but Juncker was clearly in a playful mood as he received EU leaders alongside EU president Donald Tusk and Latvian Prime Minister Laimdota Straujuma.
The dictator jibe from Juncker recalled a bitter row caused by US Republican Senator John McCain who labelled Orban a "neofascist dictator" in December last year. Orban called McCain's remarks an "attack on national independence".
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The Hungarian leader again angered EU lawmakers and officials this week by insisting that Budapest had the right to debate closing the door to migrants and reintroducing the death penalty.
"Hungarians talk straight about tough things. We don't like to beat about the bush. We are a frank people," Orban said on a visit to the European Parliament in Strasbourg on Tuesday.