Italian striker Federico Macheda sealed his move with just three hours remaining of the January transfer window, following deals earlier in the day for defenders Tom Thorpe and Tyler Blackett.
St Andrew's is fast becoming a popular destination for United loanees, with Jesse Lingard proving popular during two spells there earlier this term.
Twenty-two-year-old Macheda had previously been at Doncaster Rovers this season, where he made 15 appearances and scored three goals. He will wear the no.9 shirt at Birmingham City.
Thorpe and Blackett are both looking forward to gaining first-team experience. Blackett told Birmingham City's online TV channel: "I'm delighted to be here. It's a good opportunity for me to come to a big club like this.
"I've known Tom my whole life, we've played together for the last eight or nine years and it will be great for me and him."