The midfielder has left Aston Villa for Newcastle in a deal worth £40m. Ramsey has signed a long-term deal with the Magpies.
Ramsey told the club's website: "I'm delighted to be here. It's been a mad two or three days, but I'm really happy to be here and I can't wait to get started.
"This is a big move for me, but as soon as I knew the gaffer was interested and really liked me, it didn't take much time to decide. His track record of developing players, especially those who have gone on to become internationals here, speaks for itself.
"It was always a tough game playing for Villa against Newcastle - the team is full of energy, so athletic and the fans are so passionate. I feel my game will suit that and I'm excited to be on the other end of it now."
Toon manager Eddie Howe said: "Jacob is another fantastic addition to our squad. His qualities will bring us something different on the pitch and he fits the profile of being young and hungry to develop, but also with a wealth of Premier League football under his belt.
"Jacob offers versatility and his directness in attack and has proven to be a real threat at this level in recent seasons. He performed very well in the Champions League last season too and we believe he'll be a major asset for us."