Thorne has been on the Rams' radar since October but subsequently joined Sky Bet Championship rivals Watford, where he made eight starts.
Head coach Steve McClaren continued to monitor the 21-year-old and finally got his transfer target on Friday.
"George is a promising young player and we're pleased he will be with us for the rest of the season," said McClaren.
Thorne, who came through the youth system at West Brom, is expected to go straight into the Derby squad for the game against Birmingham at St Andrew's on Saturday.
The former England Under-19 international had loan spells at Portsmouth and Peterborough before suffering a serious knee injury playing for Albion in a Premier League fixture at Everton in January. He has been working his way back to full match fitness during his time at Watford.
Thorne has played 17 times for West Brom and made his first-team debut aged 16 during the 2009/10 season.
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- West Brom
- Sky Bet Championship