The 27-year-old centre-back arrives at Firhill from Coventry for an undisclosed fee ahead of Partick's season-opening Scottish Premiership meeting with Ross County on Wednesday.
A statement published on Partick's official website read: "We would like to welcome Danny to Firhill and wish him every success in his time at the club."
Seaborne made 53 first-team appearances for Southampton and spent time on loan with Charlton and Bournemouth before joining Yeovil upon his departure from St Mary's.
A loan move to Coventry from Somerset became a permanent switch in January and he made 16 league appearances for the Sky Blues, scoring one goal.
In September 2011 Seaborne suffered serious head injuries in an attack outside a Southampton nightclub and did not play for the Saints for the remainder of the 2011-12 season.