The defender arrives at Fratton Park on a permanent deal until the end of the season for an undisclosed fee, to provide some depth in the left-back position.
The former Arsenal man has spent two-and-a-half years with the Millers, where he’s picked up 102 appearances – including 75 outings in the Championship.
Bramall will bring his pacy attributes to a department where Connor Ogilvie is currently the only orthodox option, with Jacob Farrell out for the season.
The 28-year-old becomes the fifth arrival of the January window following loan deals for Rob Atkinson and Isaac Hayden, with Aussies Thomas Waddingham and Hayden Matthews sealing permanent deals.