The 17-year-old came off the bench with 11 minutes of normal time remaining against Athletic, and was immediately sent clean through after making a great run down the inside-left channel.
And, after a first touch to set himself, the tall centre-forward slotted a fine finish beyond Athletic goalkeeper Unai Simon to spark jubilant scenes in Barcelona.
The youngster was a surprise inclusion on the bench for the match, having only previously made two full appearances for Barcelona B.
But the talented Spanish Under-17 international’s moment of brilliance proved the difference on a night that saw chances go begging at both ends.
It’s all over in Barcelona and Marc Guiu is the hero. It finishes 1-0 and Barcelona are just one point off the top. Brilliant stuff from the youngster.
The officials have released the findings of a VAR check on that goal, which showed that Guiu was onside – just. There must have been millimetres in it, but he timed his run perfectly.
Guiu’s pulse is racing and he charges towards Simon to close the keeper down, catching his man with a wild attempt at a block. It’s a yellow card and he’s quite fortunate it was such a misdirected challenge, because that was reckless.