Merino has left Newcastle to join La Liga side Real Sociedad.
The 22-year-old has been linked with a move away from Newcastle for much of the summer, after falling down the pecking order under Rafa Benitez around the Christmas period due to a back injury.
It saw him appear for just 93 minutes from March onwards, and has now decided to end his stay on Tyneside just 12 months after moving to Newcastle on loan - before a permanent £7million deal was activated.
Merino posted on Instagram to thank Newcastle fans for their support and to describe his time in the north east as a "massive experience" that he "will never forget".
The Spain Under-21 international made 25 appearances for Newcastle in all competitions during his time with the Magpies, his highlight likely being the winner in a 1-0 win over Crystal Palace in October.
The former Borussia Dortmund star has signed a five-year contract with Sociedad, which leaves Rafa Benitez with just Isaac Hayden, Jonjo Shelvey and Mo Diame in central midfield.