Under the rules of the youth loan, Caulker will still be allowed to train with Tottenham and play reserve team matches for them. He can also be recalled after the first 28 days.
City manager Millen, who already has Danny Rose on loan from Tottenham, said: "He's very athletic, quick and has got a fantastic attitude.
"I think Danny Rose has put in a good word for us and it's good that we've got a good working relationship now with Tottenham.
"Tottenham have got high hopes for him to become a Premier League defender and hopefully we can improve him and he can do well for us."