Newcastle are thought to have payed somewhere between £5-6m for the Scotland international.
Newcastle boss Rafael Benitez was appeared to be very happy with his new signing.
"I am very pleased to bring Grant Hanley to our project to add his hard work and potential to our squad," Benitez said.
"Grant is a young player with a lot of ambition to improve and keep growing with us".
Hanley himself also spoke of his delight to move to St James Park.
"It is a fantastic opportunity to come and play at such a big club and one that I couldn't turn down," he said.
"It is an exciting step in my career but it doesn't stop there. This is just the start and I am looking forward to helping Newcastle get back into the Premier League".