The Tottenham Hotspur forward has spent this week training with Derby and has now secured a three-month loan deal at Pride Park.
He has twice trained with McClaren at other clubs and the 25-year-old, who had a spell on loan at Aston Villa last season, is happy to be linking up with the former England coach.
"I am really excited to join," Dawkins told Derby's official website. "It is a big, big club. I know the manager well so I cannot wait to get playing.
"He tried to get me at FC Twente, whether it was a loan move or something on a permanent basis I am not too sure, but that didn't quite happen.
"Then we ran into each other again at QPR, and a transfer didn't quite happen there either. But hopefully this time he has got his player!
"All the managers that you really like, you hope that wherever they go they keep you in mind, and hopefully I can now repay the faith that he has shown in me.
"You always want to play for a big club and a great manager, and I have that opportunity now, so it is time to repay his faith."
Derby boss McClaren added: "I'm really pleased we've been able to bring Simon in and add to the group we already have.
"He's someone I know and it was a 'no-brainer' getting him in to train with us and this week has been about seeing how he settled into the squad, rather than about his ability.
"He has done very well and it's great to have him on board for the next three months."