The 21-year-old, who played 12 games for the Rams on loan from West Brom last season, says the prospect of County getting up to the top flight persuaded him to join on a permanent basis.
He told Rams Player: "I'm delighted to become a Derby County player on a full-time basis. It's been dragging on a little bit but we've got there in the end.
"I was relieved when a fee was agreed. It's great to get everything done because, naturally, you have moments when you think it might not happen. Credit to the owners, Sam Rush and Derby County for sticking in there and getting a deal agreed.
"I knew, with the players going to Austria on Sunday, it was a crucial time in terms of the transfer being finalised. I truly believe this is a club on the up; I wouldn't have signed if I didn't think that Derby County could be promoted to the Premier League."
Thorne scored in Derby's play-off semi-final victory over Brighton before making an appearance in the Wembley defeat to QPR.
"I really enjoyed the loan spell last season and working with Steve McClaren," he added. "I think the style of play suits me down to the ground and a key factor in coming here permanently is the chance to play first-team football.
"The way the club has retained the key players has been fantastic. It is a sign that Derby County is building for the future. As I said earlier, I truly believe the club is heading towards the Premier League. I've left a club from the top flight and I want to get back there as soon as I can."
You can watch Derby take on Carlisle in the Capital One Cup on Monday August 11, live on Sky Sports 1HD.