They have paid £6.5m for the 24-year-old former Charlton midfielder
He said: As soon as Chelsea made it clear they were prepared to let me leave, I was keen on this move.
"I've played here a few times. I know how big the club is. I was impressed with the manager Graeme Souness. He's a major factor behind my decision.
"I was a key figure at Charlton and playing every week, and then all of a sudden at Chelsea I wasn't a regular and had to get used to being on the sidelines.
"But that has made me a stronger person and I don't leave Chelsea believing that I have failed.
"I have learned a lot there and developed as a player and I'm confident you will see that at Newcastle next season."
"I have a desire to win things and I feel that at Newcastle I can do that," he said.
"The manager has given me a huge lift, I feel wanted again and I know I can do a good job here."
Good Luck, Scotty.