The 29-year-old Spaniard has penned a one-year deal after passing a medical at the Championship club.
Poyet told the club's official website: "He is technically very gifted and has great ability.
"He is a right-sided midfielder and gives us another option. Ultimately he is going to bring extra quality to the squad and a different attacking midfield option."
Lopez will not be in the squad for Saturday's Championship clash at Burnley and will join up with his new team-mates on Monday.
Dean Hammond had earlier joined the club on a one-year loan from Southampton while Swansea striker Stephen Dobbie joined on a three-year deal for an undisclosed fee.