The 28-year-old Venezuelan has spent three season at the Baggies, making 108 Premier League appearances and scoring 24 goals.
"It's an honour to be here and I'm excited to get started with my team mates," Rondon told the club's official website.
"I'm focused on the season to help my team mates to get the points to do well this season."
Rondon is Newcastle's sixth summer signing, following the arrivals of Martin Dubravka, Kenedy, Ki Sung-yueng, Fabian Schar and Yoshinori Muto.
The Magpies open their PL campaign at home to Tottenham Hotspur on Saturday, 11 August.