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 19-0 professional record including 16 previous knockouts, according to Bleacher Report.
Fonseca, 26, who was born in Nicaragua, entered at 25-2 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.