Miller will add a wealth of experience to the squad and joins on a one-year deal following his release from Huddersfield.
Navarro has also joined on a free transfer after he was released by Brighton and has penned a two-year deal.
Miller will link up with former Huddersfield midfielder Gary Roberts, who also made the switch to the County Ground.
Defender Jay McEveley later became Swindon's sixth signing of the summer.
The 27-year-old spent the end of last season on loan at the County Ground and has now made the move from Barnsley permanent on a two-year deal.
McEveley has over 200 league appearances to his name and was an instrumental member of the League Two winning team in the latter stages of last season.