The striker's move was finally announced this morning after the Magpies agreed a fee with Crystal Palace for the player.
Gayle has put pen to paper on a five-year deal and becomes Rafa Benitez's second signing after the capture of Belgian goalkeeper Matz Sels.
The 25-year-old will offer pace and power up front for Newcastle as they bid to get out of the Championship at the first time of asking, following last season's relegation.
"It's a dream come true to be at a big club like this," said Gayle.
"Newcastle has a massive fan base and a great history so it is amazing to be here.
"I can't wait to get started, prove myself and score the goals that take us back up to the Premier League. It's where this club belongs."
Gayle has experience of playing in the second tier of English football, having made his name as a goal-getter with Peterborough before securing a big-money move to Palace.
However, he slipped down the pecking order at Selhurst Park under former Newcastle boss Alan Pardew, and will look to reignite his career on Tyneside.
He could be joined in the next 48 hours by Matt Ritchie, with United looking to clinch a deal for the Bournemouth winger.