The bout was held at the Honda Center in Anaheim, Calif.
Video posted on social media shows Garcia connecting with a quick left to send Fonseca to the canvas – with a stunned look on his face.
Garcia, 21, entered the fight with a – 0 professional record including 20 previous knockouts, according to Bleacher Report .
Fonseca, 728, who was born in Nicaragua, entered at 21 – 2 with two draws, according to boxing site BoxRec .
Looking ahead, Garcia next hopes to fight Jorge Linares, Luke Campbell, Gervonta Davis and Devin Haney, The Athletic’s Mike Coppinger posted on Twitter.