Kay, 34, who joined Bury from MK Dons in the summer of 2016, has left Gigg Lane to join Michael Brown's relegated League Two side on a two-year contract.
Vale's first signing of the summer was the return to the club of Tom Pope.
Defender Joe Davis is another ex-Vale player to have returned to Burslem.
Vale boss Brown has also brought in Sheffield United defender Graham Kelly, Bristol Rovers winger Cristian Montano and goalkeepers Rob Lainton and Sam Hornby.
Kay has made 620 career appearances, the highlights of which were twice winning promotion with Barnsley in 2004 and 2006, when he scored the winning penalty in the League One play-off final shootout win over Swansea.
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Port Vale have made their seventh new signing of the summer by bringing in experienced former Barnsley, Tranmere Rovers, Huddersfield Town and MK Dons defender Antony Kay from Bury.