The 19-year-old who turns 20 next month, has previously been on trial with Arsenal, Chelsea and Ipswich.
But he has now put pen to paper on a two-year-deal at The Hawthorns which includes the option of a further year in the club’s favour.
Described as an energetic and powerful central midfielder, Castro joined up with the Baggies for a trial at the end of last season – and played for the under-23s against Wolves.
He was then invited back for the start of pre-season.
And, according to a social media post from his agency Eyeconicsports, the teenager has since gone on to impress first-team coach Valerien Ismael in training.
The post said: “We are proud to announce the signing of Quevin Castro who has officially signed with WBA on a two-year deal.
“He is a very powerful, quick and technically gifted midfield player who works extremely hard.
“After a two-week successful trial at the end of last season, he was offered a deal with Baggies under-23s.
“He has come in for pre-season and done extremely well – joining the first-team squad in the past two weeks.
“He has really impressed the manager and the first-team coaching staff.”
The post confirmed Albion have paid a compensation fee to an unnamed Portuguese side to land Castro, who is likely to begin his career at the club with the under-23s.
“We have been in intense negotiations with a Portuguese club for a training compensation fee for several weeks,” it added.
“Finally we have managed to come to an agreement. Such a proud moment and we are so excited for the future.”
Castro’s journey to The Hawthorns has been an interesting one.
The teenager grew up in Norfolk and had spells in non-league with Bury Town, Mildenhall Town and Thetford Town.
He then had a trial with Ipswich and would have signed for them but the League One side were unable to agree a compensation fee.
But after that move broke down, Castro was invited to train with both Arsenal and Chelsea.
And Danny White, his former manager at Thetford, is adamant the midfielder has what it takes to play at a high level.
“First and foremost he is so dedicated and committed,” he said. “He trains constantly and his attitude is first-class – he just wants a break.
“But the problem with that is obviously you’ve got all these professional clubs where lads are getting released and you don’t really see them getting into the big clubs or even falling down the leagues.
“I personally think he has got all the potential in the world.
“If he gets with some world-class coaches and they work on him then he could become a very, very good talent.
“I expect him to play high.”
After his trial at Arsenal, Castro spent time with Portuguese second-tier club Varzin, where he trained with the first team while representing their B team.