After defeating Bodø/Glimt and winning the Norwegian Eliteserien last season, Viking made its debut in the world's largest club tournament.
But Wednesday's opening match ended in defeat.
Away to German side Stuttgart, they lost 1-3 after home forward Ermedin Demirović scored a hat trick in the first half.
It won't get any easier for the Norwegians in the rest of the Champions League. In their second match in the league phase, they face another German team, none other than Bayern Munich.
In the parallel match between FC Barcelona and Feyenoord, the Spanish champions won 5-1.
