The Queens Park Rangers midfielder will spend the next month at The Den, but has not ruled out extending the agreement.
The 35-year-old told the South London Press: "I'm absolutely chuffed about sorting it out, and QPR have been as good as gold about it. I have signed for a month and it is up to me to produce the goods. And at the end of the month, we will sit around the table and see what can be sorted out.
"I had tentative enquiries from other clubs, you just have to look at the fact that Harry Redknapp has signed two midfielders in the last fortnight alone to know people would be interested. But I have always had a soft spot for Millwall.
"But it is also important for me to get out and play some games. I do not want to be sticking around somewhere and not playing games. I want to be involved and hopefully I will get a chance to do that at Millwall. That would be perfect."
Derry signed a one-year contract extension with QPR in January.