The 19-year-old has agreed a new six-month deal at the Liberty Stadium after impressing in a shorter stay at the end of last season.
"We are delighted to have him back," Martinez told the club's official website.
"His attitude last season was excellent and we enjoyed watching and working with a player of Premier League potential.
"He always made it clear that he wanted to come back and join our Championship campaign and we are looking forward to seeing him perform at the next level.
"He's got balance, strength, sharpness and a great eye for goal. He's worked hard over the pre-season and will be back a stronger player.
"He's a great addition to the squad; he's a completely different player to those we've got and will provide added competition to the squad."