The 29-year-old made 62 league appearances for Cardiff, before being released at the end of last season.
Plymouth boss Peter Reid told BBC Spotlight: "He was a fine player for Plymouth in the past and he's had a decent career."
Reid added: "He's got good experience and has got that balance on the left side.
"It's a chance for Tony to have a look at us, and me to have a look at him."
Capaldi spent four years at Home Park, making 153 appearances after joining on a free transfer from Birmingham.
Should Plymouth re-sign Capaldi?
During his time in Wales, Capaldi played in Cardiff's FA Cup final defeat by Portsmouth in 2008.
The defender has now been selected for Plymouth's pre-season tour of Holland.
Fellow trialist, Bermuda international goalkeeper Freddy Hall, will also travel with the Argyle squad to Holland.
Meanwhile, defender Chris Barker has been released by Plymouth and will not travel to Holland.
Barker, 30, spent two years at Home Park, making 54 league appearances for the club.
Plymouth Argyle squad for Holland tour: Kari Arnason, Onismor Bhasera, Yala Bolasie, Tony Capaldi, Chris Clark, Georgie Donnelly, Karl Duguid, Carl Fletcher, Freddy Hall, Liam Head, Reda Johnson, Damien Johnson, Sean Kinsella, Romain Larrieu, Steve MacLean, Joe Mason, Bondz N'Gala, Craig Noone, Jim Paterson, Rory Patterson, Anton Peterlin, Marcel Seip, Luke Summerfield, Krisztian Timar, Jordan Trott, Bradley Wright-Phillips.
Source: BBC Sport
Source: BBC Sport