The 30-year-old - formerly of Aston Villa, Sheffield United and Derby County - joined Forest on a free last summer and went on to make 31 league appearances in his debut season, scoring twice.
Brewers boss Nigel Clough told the club's website: "We know Jamie well. He's a current Northern Ireland international and he will give us another very good striking option.
"We are delighted with the business we have done so far in this window and if this squad stays fully fit, it gives us a chance up until January. It means we have four internationals now in the squad which is also a sign of how far we have come as a club.
"I think it's good to add some more real Championship experience. [Ward] can be a fiery character but I don't think a bit of edge will go amiss in the next few months. A bit of aggression all over the pitch is probably what we need."
Burton head into the international break 13th in the Championship table after two wins and a draw from their opening five fixtures, while Forest sit just outside the playoffs in seventh.