The 32-year-old veteran defender has signed a two-year-deal with the Championship outfit, marking the first signing under new manager Gary Rowett.
Davies made 29 appearances for Hull in the 2016/17 season where they were relegated from the Premier League despite Marco Silva's best efforts after taking up the reigns from January onwards.
4 years at Hull City now over
Davies was at the club for four years having previously played for Luton Town, West Brom, Aston Villa and Birmingham City. The centre-back has featured in 460 games as a footballer. In that time, Davies has netted 26 goals.
He has long been a crucial figure at Hull City, providing a linchpin at the back as well as an authoritative and experienced head. Consequently, it will be a monumental loss for Hull City, particularly with their recent relegation to the second tier.
In contrast, it's a coup in many ways for Derby County. Finishing ninth, thirteen points outside the play-off places, the club secured a sturdy finish under Rowett - who took over after Steve McClaren was sacked. He will be crucial to any hopes they have of mounting a challenge on the top spots in the Championship.
Davies joins Derby for undisclosed fee
Speaking to Derby after making his move to the club, Davies said:
"I believe, under this manager for a full season, the squad we've got is capable of promotion or at least the play-offs."
"I believe that, at my age, people start thinking that you want to wind down and look for a bit of security but that is not the case with me; if I'd have wanted that, I would have stayed at Hull," said Davies.
"This is an opportunity for me to reinvigorate myself. I've enjoyed my time at Hull and there are a lot of people that I'm going to miss but I want to do the best for my career and I believe that is with Derby County," he concluded.