The 29-year-old has become the Championship newcomers' fourth new signing and agreed a two-year contract.
Broadfoot, who made more than 100 appearances for Rangers, was a regular in the second tier last season with Blackpool.
"It has taken a little while to get done because of the significant interest there was in Kirk," said Millers boss Steve Evans.
"There will be some disappointed managers today, we had to work ever so hard to get him here and I am delighted to have got the deal over the line.
"He is an experienced professional who has played at the highest level with Glasgow Rangers. He has played in an UEFA Cup final; he has significant experience in the Champions League and he has won the Scottish Premier League several times.''
Broadfoot collected a host of honours with Rangers, including three league titles and a runners-up medal in the UEFA Cup.