The 19-year-old has signed a five-year contract and will join Borussia Dortmund on July 1, the Bundesliga side have confirmed.
Merino said: “Borussia Dortmund are one of the top clubs in Europe.
“I’m honoured that I’ll be playing for them soon, they represent a very special brand of football and play in an impressive stadium.”
Merino featured for the Spain Under-19 squad who won the 2015 European Championship in Greece.
