The 66-year-old South African, who stepped down as Edinburgh boss in September, has previously coached Northampton Saints and Ulster.
He arrives with Bristol bottom of the top flight, six points below Worcester.
"Alan is a hugely experienced operator who understands the game," head coach Mark Tainton told the club website.
"With seven games remaining, it's vital that Bristol rediscover our form from over the Christmas period and Alan's arrival as a consultant should give everyone a boost."
Meanwhile, Tainton - who has worked as interim boss since Andy Robinson's departure in November - is to become Bristol's chief operations officer at the end of the season, overseeing the club's recruitment policy.
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Connacht boss Pat Lam has already been appointed as Bristol's new head coach from the start of next season.
Chief executive Steve Gorvett will also step down at the end of the current campaign.
Gorvett has been with Bristol for 12 season, initially arriving in 2005 as finance director before taking on the chief executive role in 2008.
"I took the decision to step down at the end of the season after 12 enjoyable years at Bristol," Gorvett said. "It's been a period of huge transition for the club and we have faced a series of significant challenges along the way.
"At the end of the season, we aim to still be in the Premiership with a world-class head coach already committed, a stadium that we are proud to call home and a state-of-the-art training ground on the way."
Former Edinburgh head coach Alan Solomons has been appointed as a coaching consultant at Premiership club Bristol for the rest of the season.