Ritchie, 25, has agreed a five-year contract after the clubs agreed a fee, reported to be around £10 million, for the Scotland international.
“It’s an unbelievable feeling to come to a football club like this,” said Ritchie.
“I loved it at Bournemouth. I had a fantastic time there and it had a huge impact on my career. But when a club like Newcastle comes calling, I couldn’t pass up this opportunity.
“It’s fantastic to be here and I’m really looking forward to the challenge of getting the club back in the Premier League.”
Bournemouth are understood to have rejected a bid from West Ham last month for Gosport-born Ritchie, who completed his Newcastle medical on Thursday night.
He was a near ever-present for Bournemouth last season, the club’s first in the Premier League, scoring four goals in 37 appearances, while he netted 15 for Eddie Howe’s side as they won promotion from the Sky Bet Championship in the previous campaign.
Newcastle boss Rafael Benitez added: “We have been following some wingers for a while and Matt was one of the best prospects for this team.
“He has great ability and pace, and as soon as we knew that Andros Townsend could be leaving, our priority was to finalise this deal.
“We have signed an exciting winger who has experience in the Championship but has also proved that he can step up to the top level. I am delighted that he is now part of our squad.”