The 16-year-old put pen to paper on a United deal in July but has been waiting for international clearance.
And he finally confirmed the move on Instagram when he wrote: "Very happy and proud to announce that I am a new Manchester United player, a dream come true. It's time to work, come on Red Devils!"
Puigmal, 16, is a Spain youth international and was named most valuable player at the Mediterranean International Cup.
He was pictured alongside Under-18s coach Kieran McKenna and United great Andy Cole.
He has already played in friendlies for United, scoring in the U18s 6-0 win over SV Ebensee in Austria.
United also bolstered their academy this summer with the signings of Di'Shon Bernard from Chelsea and Aliou Traore from PSG.
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