Davies had been on a season long loan during the last campaign with the option of a summer transfer.
And Villa have taken advantage of the clause to seal a deal worth in the region of £9m.
The centre back is currently recovering from an Achilles injury sustained against Arsenal in March.
He is expected to be back at the end of September and hopes to face his former club at the Hawthorns on the 20th.
Davies knows he will have a fight on his hands to regain his place in the team after Zat Knight impressed towards the end of last season.
But if Curtis can rediscover his best form an England call could be on the cards at some point during the 2008/09 campaign.