The 20-year-old arrived in Scotland on Tuesday morning and could feature in Wednesday's Premier League clash against Aberdeen at McDiarmid Park.
Saints manager Steve Lomas said: "We have taken young Michael Doughty from QPR. They have kindly let us have Michael. "Michael's a very silky midfielder who is good on the ball and can play in various positions across the middle."
Doughty has made one first-team appearance for QPR off the bench but has added top-team experience from loan spells at Aldershot and Crawley. He signed a three-year deal with QPR in August.