The 30-year-old joined Championship side Leeds in January 2016 from Brentford and made 246 appearances over a six year period at the Bees.
Diagouraga's professional career began at Watford, and he has had spells playing for Swindon Town, Rotherham United, Hereford United, Peterborough United, Portsmouth and in last year's campaign, Ipswich Town.
The Frenchman had a short trial with Scunthorpe United after leaving Leeds, with Bristol City, Barnsley and MK Dons also interested in the midfielder.
The deal is subject to the usual paperwork requirements being completed, after which Diagouraga will wear the Pilgrims' number 32 shirt.
Plymouth Argyle currently sit at the foot of the League One table, having gained only five points from their first 12 games.