Rondon scored 24 Premier League goals since joining the Baggies three years ago, but couldn't prevent West Brom's relegation to the Championship last season.
"When we were looking for options up front, we knew how important experience would be. Salomon has been in the Premier League for a while and he knows how demanding it is," Newcastle manager Rafael Benitez said in a club statement.
Gayle shone in helping Newcastle gain promotion back to the Premier League in 2016/17, but scored just six times in 35 appearances in the top flight last season.
Benitez and the Newcastle fans' frustration at owner Mike Ashley's refusal to match the big spending of their Premier League competitors has continued for another transfer window as Ashley refused to meet West Brom's asking price for a permanent transfer.
However, Rondon is a fifth summer recruit after the arrivals of Martin Dubravka, Kenedy, Ki Sung-yueng, Fabian Schär and Yoshinori Muto.
"The clubs were talking for a long time and now I'm focused on the season to help my team mates to get the points to do well this season," said Rondon.
"It's one of the biggest clubs in the Premier League and I knew about the interest of the gaffer, so I'm happy to be here."