Agent David Canter made the announcement on Instagram:
Terms of the contract were not disclosed.
Thompson becomes the second veteran free agent added by the Chiefs on the eve of training camp, joining guard Kelechi Osemele, who agreed to a contract with the team after Laurent Duvernay-Tardif opted out of the 2020 season.
Thompson provides insurance for the Chiefs at free safety with Juan Thornhill recovering from a torn ACL, which he suffered last December.
The Seattle Seahawks waived Thompson on May 31 with a failed physical designation. His 2019 season ended after six games because of a shoulder injury that required labrum surgery.
Thompson, 25, was a fourth-round pick by Seattle in 2017 and was up and down in 16 starts over the past two seasons after taking over for Earl Thomas at free safety.
He made interceptions in critical moments in consecutive October victories but also had lapses in coverage that resulted in long completions, including a 50-yarder in the Seahawks’ loss to Baltimore that coach Pete Carroll lamented as a “big mistake.”
Thompson no longer projected as a starter at free safety for the Seahawks with Quandre Diggs taking over at the position.
The Chiefs’ depth chart at safety is headlined by star strong safety Tyrann Mathieu and includes veterans Thornhill and Daniel Sorensen. Kansas City also selected rookie L’Jarius Sneed in the fourth round of the 2020 NFL draft.
ESPN’s Brady Henderson contributed to this report.