The 22-year-old will go straight into the Pilgrims' squad ahead of their game away to Aldershot in League Two on Saturday.
Cox began his career at hometown club Leicester and progressed through their youth system. He was loaned out to Yeovil in 2009 before being released by Leicester.
Cox then headed north of the border and signed for Scottish outfit Inverness, making 63 appearances and helping Terry Butcher's men to promotion to the SPL in 2010.
He joined Paolo Di Canio's Swindon in January 2012 but only made seven appearances for the Robins.
Cox went on loan to Oxford at the start of this season, playing 14 times for the U's before returning to the County Ground.