Giannakopolous fly into England on Wednesday and Wanderer's boss Sam Allardyce completed the signing of the Greece international.
"Stylianos is a highly-rated player in his homeland and is looking forward to playing in the Premiership with us," Allardyce told the club's official website.
"I have been tracking his career for quite some time and I am confident that he will add a new dimension to our squad.
"His main attribute remains his ability to make late dynamic runs into the penalty area and his flair to shoot accurately from range. He is an attacking midfielder who knows how to score goals."