The Magpies have reportedly agreed a fee for the 21-year-old and he is set to join them on a permanent transfer.
The Spaniard has only been with Dortmund for one season, after joining them from Osasuna.
Merino failed to settle in Germany and now, according to Sky Deutschland, he is in England to undergo his medical and complete a move to Rafa Benitez’s side.
The deal will be a coup for Newcastle, who reportedly thought they would only be able to land the Spain under-21 international on loan.
However, they have persuaded Dortmund into selling and a deal should be wrapped up by the end of the week.