Harley has agreed a three-and-a-half year deal with the Championship promotion chasers but will spend the remainder of the current campaign back at St James` Park as part of the deal.
The playmaker's contract was due to expire at the end of this season, with a number of clubs circling for the 25-year-old's signature during the January transfer window.
Harley - who has scored 11 goals this season - will miss Saturday's clash against Walsall but will be available for the Grecians' trip to Swindon next weekend.
The switch to Swansea will end his three-year association at Exeter, who he joined in 2007 from non-league side Weston-super-Mare.
Source: PA
Source: PA