The England U21 international left Liverpool a year ago for Salzburg, but struggled to settle in Austria.
Clark had been linked with a return to former club Newcastle United, but has now joined Championship Derby.
He told Derby County TV: "Once I found out Derby were interested, I had a chance to speak with the manager and the staff. With his goals and the way he is aligned with me, it seemed right.
"(The friendly against Salzburg) was around the time when I knew that Derby were interested. It was a weird one after speaking to the coaching staff, the club and then playing against them as well!"
On manager John Eustace, Clark added: "I think he knows that I'm at a point where I want to prove people wrong, and I'm hungry to play football again, especially being back in the UK. It's a great league and I just want to kick on here."