The deal, still subject to international clearance, will see Costa join the club’s development squad.
Richard Hughes, a member of Bournemouth’s recruitment department, told the club website: “Filippo is a left-back with very good quality.
“He has good delivery and has a good engine. He will fit the mould of a Bournemouth full-back because his main attributes are offensive.
“I can’t be neutral but, from all the football I have seen, Bournemouth are the best club for coaching full-backs to play in the manner I think they should.
“Filippo has the attributes to do that. He’s a good younger player but I don’t want to put too much pressure on him because there’s clearly a lot of issues for him to deal with. If we allow him to develop, I’m sure he will fulfil that potential here.”