The 22-year-old joined Newcastle last season on an initial loan deal from Borussia Dortmund, before the club signed him on a permanent basis in October 2017.
Merino made 25 appearances in all competitions for the Magpies, scoring once and assisting on one other goal.
A niggling back injury hampered Merino's season on Tyneside which initially appeared to be one of such promise, as his skill on the ball and passing vision delighted the Toon army.
However, Merino struggled to make it back into Rafa Benitez's starting XI, as Mohamed Diame was preferred to partner Jonjo Shelvey in the middle of the park.
Merino's transfer to Real Sociedad won't come as much of a surprise to Newcastle fans, as rumours of a deal for the Spaniard had been rife for some time.
And while the fee for his move to the Spanish club is undisclosed at this time, it's hoped that those funds will be put back into Benitez's transfer kitty to help bring in more players.
Speaking to the Chronicle, Benitez admitted he knew it was inevitable that Merino would be sold.
“He is a young player and he struggled to get back into the team because [Mo] Diame and [Jonjo] Shelvey were doing so well.
“That meant Merino didn’t get much game-time, then we had three or four Spanish clubs who were interested in him.
“In the end it was something we couldn’t control because he decided to go.
“We knew that could happen, so we had to be prepared - and in the end we were.”
Earlier today, Newcastle confirmed the season-long loan of Kenedy from Chelsea. Toon fans will now hope that the club can push on and use the funds from Merino's sale to bring in more faces to the club.