Sanchez, 31, joins the Reds from Deportivo Lugo where he appeared 42 times in the Segunda A last season.
Sanchez is part of a growing Spanish influence at Adelaide United under former Barcelona youth coach Josep Gombau and joins midfielder Isaias Sanchez and forward Sergio Cirio.
"We need a player that can play in front and also defend, a player with experience, especially because the age of our team is so young," Gombau said.
"We are trying to play the Spanish style and to bring a player from another league who doesn't play this style it makes it difficult. His previous coach played in the same style so it is easier for Pablo to adapt here."