Finnish Under-21 defender Carl Jenkinson became Arsenal manager Arsene Wenger's first signing of the?summer on Wedneday when he joined from League One side Charlton.
The 19-year-old mostly plays at fullback but can also fill in at centreback which he has done with Charlton since making his first team debut late last year.
"I hope supporters will understand why I can't turn down such an opportunity, but Charlton will always have a place in my heart," said Jenkinson, who supported Arsenal as a boy.
"I will always be in Charlton's debt for the training they have given me since a young age.
"I'm grateful to the club's academy and all the coaching staff, so I'm delighted the two clubs have come to an agreement."
Jenkinson will be one of several new arrivals at Arsenal this summer as Wenger has vowed an overhaul of the squad after a sixth successive campaign without a trophy.