He was a huge fans' favourite at Loftus Road, playing 15 games in three months - and many fans hoped he would be signed permanently.
The pacy wide man made his debut at left-back for QPR in the 1-1 draw at The Den in September.
He said: "I had a call from my agent on Thursday saying Millwall were interested and jumped at the chance.
"It will be a big challenge and I hope I can make my contribution. I thought I could not let this opportunity go by to play regular first-team football and help this club.
"It will be tough getting out of the relegation zone but I hope I can play my part and keep us up.
"My best position I feel is left midfield but I can do a job at left-back as well."
Dyer had been coming to the end of his contract at the Hawthorns. "So coming to The Den will give me the chance to kick-start my career," he said.
"I would like to thank the manager David Tuttle for giving me this chance to play for Millwall.
"I really need to be playing first-team football and that was not happening at West Brom.
"Moving down to London will not be a problem - I am used to it here after my loan spells. I like to think I
l l continued from back page have some pace and can cause defences problems. I am looking forward to being able to do that for Millwall."
Dyer could make his debut at Cardiff tonight, alongside Dutch striker Berry Powel, signed last week.
Tuttle said: "We are going into every game to try and win. We are trying to get out of the bottom three but they are trying to get into the play-off places so it is going to be a hard game."
Tuttle was yesterday still trying to sign players but was becoming increasingly pessimistic about his chances of adding to the squad. l Former manager Colin Lee quit his job as director of football on Friday after six months at the club.
The ex-Walsall boss had become increasingly frustrated at the knock-backs he had received in trying to sign players for Millwall.
Chairman Peter de Savary said: "We would like to thank him for his endeavours and wish him every success in the future."