Redknapp brought the Croatia midfielder to Loftus Road on loan last season and the pair also worked together at Tottenham and Portsmouth.
The Dynamo Kiev playmaker is delighted to be coming back to London, as he told the QPR website: “It’s a great feeling to be back, it feels like home. I felt a special emotion with the club and the fans.
“The fans were phenomenal last season. I had a great relationship with them and hopefully it’s only going to get better.
“I really feel happy here, at a club where I really want to play, and I play with all my heart. I can’t wait for my season to start with QPR.
“I believe I still have something to prove, for myself and for all the fans. The Premier League is a lot more physical than it was in the past but I’m looking forward to it. I have no doubt this club is going to be in the Premier League for a long time.”
Redknapp added: “Niko is a class player. I’m a huge, huge fan of his and really pleased to have him back here.
“He showed QPR fans last season what he can do and I actually believe he will be even better this year for us in the Premier League.
“He will have better players around him now, and the better players he plays with, the better player he is himself.
“Niko’s a real talent, and I am really pleased with the business we have done throughout the summer window. Getting Niko and Sandro in is the icing on the cake for us."