Thorne, who completed a permanent switch from West Bromwich Albion on Saturday, believes that Derby are capable of succeeding in the Championship.
"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," he told the club's official website.
"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 two goals in eight appearances for Derby during a loan spell last season.