A five game unbeaten run ended on Tuesday evening, as Valérien Ismaël suffered his first loss as Reds head coach in the Sky Bet Championship.
Cardiff City took the lead in the fourth minute, before doubling their advantage shortly before half-time. Harry Wilson added a third to condemn the Reds to defeat in the Welsh capital.
Barnsley have a chance to react immediately, as they visit Pride Park to face Derby County on Saturday.