The Swede, who can play up front or on the wing, agreed a four-year deal to join the Black Cats from Gothenburg.
And the 19-year-old, who picked up a Swedish Cup winners' medal after Gothenburg's penalty victory over Djugardens, is relishing the challenge ahead.
"It feels very good, I've looked at the facilities and it's great here," said Moberg Karlsson.
"It's a great club, I love the Barclays Premier League; it's the best league in the world and Sunderland is a big club in England so it wasn't a hard decision.
"Winning the Swedish cup meant a lot, it means a lot to win trophies and I play football to win trophies so it was a great experience.
"I come to a club with great players so I get to train with great players and a great head coach so I think it's a good step.
"Of course I want to play as much as I can but also to train hard and become a better player, to train with good people.
"I can't wait, now I'm here and I see the facilities and the training ground - my legs want to move, I want to play football, it's fantastic.
"I want to show them what I can do and hopefully soon I will play."