A deal for the prolific forward, also known as Kike, was agreed a few days ago and he has now completed the formalities on the move after linking up with the squad at their pre-season training camp in Marbella on Monday.
Kike, a former Spanish Under-20 striker, has scored 48 goals in 150 league games for Murcia, and he bagged 23 goals as Murcia finished fourth in the Segunda Division.
The 24-year-old has a good record in front of goal, and is seen as an exciting signing ahead of the new season.
The striker will become the club's second signing of the transfer window, joining fellow Spaniard Tomas Mejias.
"We're delighted," said Boro boss Aitor Karanka. "He's a very good player, and a very good person. He scored a lot of goals, and he will help us.
"He's come in and I have a very good feeling, but I don't want to put pressure on him.
"I'm sure he will be very comfortable here. He was given a good welcome."