The Senegal striker quit West Ham after activating a relegation clause in his contract.
He signed a three-year deal, worth £4million, after talks with Toon boss Alan Pardew, who outlined his plans for next season.
Ba, 26, said: "I've come here as I feel the club can reach a higher level than most of the other clubs in the Premier League.
"Things will go from strength to strength. This is a club that will improve and I want to be somewhere like that.
"When I spoke to the manager, I was happy with the ambition Newcastle are showing.
"We talked and they spoke about where they want this club to be, the players they want to sign and I was pleased to hear that they want the same things I want to achieve.
"I came here because of the club, it's massive and with a great history. For me, it was a simple decision. I'll get better here."
Ba hit seven goals in 12 games for the Hammers and admitted he had mixed emotions about leaving.