Tella, 17, has been at the Gunners' academy since the age of seven but will now make the move to the south coast after impressing Saints' staff. The winger turned out for Southampton's under-18 side as a trialist against Leicester City last week and has now confirmed a permanent switch, passing on his goodbyes to Gunners' teammates and staff via Instagram.
"After a great 10 years at Arsenal, sadly I have to move on to a new challenge," he wrote. Thank you everyone who has helped me over the past 10 years to make me into the player I am. I will miss the boys and hope they have a great career.
"However, I am delighted to sign with Southampton FC. Can't wait to see what the future holds for me at this club. Thank you God for allowing me to achieve this goal and also everyone who has supported me."
Tella, described as an "explosive midfielder" who has "impressed on the wing and in central midfield" on Arsenal's official website, has made nine appearance for Arsenal's Under-18 side this season, scoring one goal.
The winger has been actively looking for a new club since the turn of the year with his scholarship deal set to expire in the summer. He also had a trial with Reading in February before he was snapped up by Southampton.