Newcastle signed Merino on loan from Borussia Dortmund in the summer transfer window.
The Premier League club have activated the option to secure the services of the 21-year-old midfielder on a permanent deal.
Former Newcastle centre-forward Quinn has taken to Twitter to give his reaction to Merino committing his long-term future to the Premier League club.
Merino is a very good and talented young midfielder, and the 21-year-old has been playing well for Newcastle this season.
The Spain Under-21 international midfielder has started six matches in the Premier League for the Magpies so far this campaign.
The former Osasuna star is still very young and is developing, and working under manager Rafael Benitez is only going to make him better in the coming years.
Newcastle won automatic promotion from Championship to the Premier League at the end of last season.