The measure has been at the heart of the welfare reforms being imposed by the Coalition Government and Mr Rennie has previously defended it.
But, in a shock climbdown today, Mr Rennie said it should be scrapped. Asked if the “tax” should go, he told the BBC: “I don’t think it should stay.”
It came as the Scottish Liberal Democrats gather in Aberdeen for their annual conference which will be addressed by national leader Nick Clegg.
Mr Rennie previously described the changes to housing benefits as “tough”, but had not signalled that he was in favour of scrapping the policy altogether.
But he said in a BBC webcast today: “The principle behind [the spare room subsidy] I can understand, but to be honest I don’t think it is working as it was intended and I think it should just go, and it should go quickly.”