Poyet tried to sign Harley in January only to lose out to Swansea for the winger's signature.
Harley, 26, never played a game for Swansea after he was allowed to return to Exeter on loan last season.
Swansea boss Brendan Rodgers has decided to let Harley move on as he is well down the pecking order at the Liberty Stadium
Poyet was delighted to land Harley and believes he will be a valuable addition to his squad.
"We know what he is about, he is a player who likes to get on the ball and make things happen," Poyet told the club's official website.
"He has a great range of passing - and he will complement the players we already have here - but his first challenge will be to earn his place in the team, as the team has made a great start in the Championship."
Harley is relishing joining the Championship new boys and is hoping he can be a success at the Amex Stadium.
"I'm really excited about the move, the stadium and the type of football they play. I'm a player that likes to get on the ball, create and score goals."