The Rams have signed Thorne on loan for the rest of the season and he is expected to go straight into their squad for the game against Birmingham at the weekend.
Manager Steve McClaren told Derby County's official website: "George is a promising young player and we're pleased he will be with us for the rest of the season."
Thorne has been a product of the West Brom youth academy and has played for England at an Under 16, 17, 18, and 19 level.
Although the 21-year-old is a West Brom player he has had four spells with three different clubs including Watford before joining McClaren's men.
Along with this news, Derby has announced that Keane will be concluding his loan spell and returning to Manchester United.
After the Rams' win over Yeovil on Tuesday, McClaren had talks with the player about his future at the club where Keane expressed his wish for regular football.
McClaren said: "We're disappointed to lose Michael but we couldn't guarantee him games and he wants to play regular first-team football right now.
"He did well for us when he was in the side and we wish him well for the future."