The 26-year-old striker, who has scored eight times in 33 appearances this season, goes straight into the Robins' squad for today's game against Swindon Town.
"We've got three strikers and we've wanted four for some time," City manager Steve Cotterill is quoted as saying by the Bristol Post. "We've had to wait a bit for Martin to see what his situation is for him.
"Martin's a very hard-working striker who will offer us another option in attack. We're delighted to have him."
Patterson joined Huddersfield on a free transfer in June 2013.