Warnock, 30, is yet to make a first-team appearance at Villa Park since Paul Lambert's arrival as manager in the close season.
Wanderers boss Owen Coyle initially tried to bring in the former Liverpool and Blackburn defender before the summer transfer window closed and is pleased to have got his man as the Trotters look to boost a slow start to the Championship season that has seen them take seven points from six matches.
"We tried to get him to the club before the summer transfer deadline but it never came to fruition, so I'm delighted that we have now been able to bring him in," Coyle told Bolton's official website.
"He has a terrific pedigree and has moved for large sums of money in his career, so we know his quality.
"He has the right character and it's important that we bring those attributes into the group. He ticks all the boxes and when I spoke to him you could tell how keen he was to come here. I have to say there was a lot of competition for his signature. We beat off a host of clubs in the Championship, so we are delighted to get him in."