The 27-year-old centre-back arrives at Firhill on a free transfer ahead of Partick's season-opening Scottish Premiership meeting with Ross County on Wednesday.
A statement 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 after leaving 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, but his contract was cancelled by the League One club last week.
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.