The midfielder comes from a West Ham family and penned his deal this week.
“I was buzzing when I found out the news,” Hargan told the club's website. “To sign my first professional contract for the Club I’ve supported my whole life is surreal. It’s something I’ve dreamed of since I was a kid.
“I had a meeting with the staff to find out if I’d been offered a deal. I was obviously nervous going into it, you hope for the best, but you never know. When they told me, my face just lit up. I was over the moon.”
West Ham chief Mark Noble also said: “Riley has really kicked on in the last three or four months. He’s become one of the first names on the team sheet for the U18s and finished the season in a rich vein of form.
“He’s earned this contract the right way, showing consistency, drive and character. When we sat down with the U18s and U21s staff and reviewed his progress, the decision was unanimous. He’s forced our hand in the best way, and now we’re all looking forward to seeing him continue his journey as an U21s player.”