The 43-year-old is a former Saints player, who was previously No 2 to Wright during a highly-successful spell at McDiarmid Park.
Davidson was capped 19 times by Scotland and had coaching stints at Stoke City and Dunfermline Athletic before being appointed assistant manager at Millwall.
The former Leicester City, Blackburn Rovers and Preston North End full-back now returns to Perth ahead of the new Premiership season starting on August 1.
