The 30-year-old has been training with City for the past 10 days and joins on a free transfer, having been released by Portsmouth at the end of January.
Bristol City manager Sean O'Driscoll said: "Brian was available and financially fits into the model going forwards. His personality will add something positive to the group.
"He's got Championship experience and is technically very good."
Former Swindon, Barnsley, Sheffield United and Reading player Howard joined Portsmouth on the eve of the current season and captained their side in League One, playing 26 games.
However, Pompey's financial problems meant they had to release him earlier in the year.