The FCA said that there was "insufficient basis for taking any enforcement action" but added that it would raise several matters uncovered by the investigation with the life insurer and pensions provider.
The regulator had been investigating Scottish Widows and five other insurers since March 2016 after conducting a review of their treatment of longstanding customers.
Scottish Widows is the second insurer to escape FCA penalties after the regulator closed its investigation of Police Mutual in September 2017.
The FCA said investigations are continuing into Prudential PLC (LON:PRU), Chesnara PLC's (LON:CSN) Countrywide Assured, Old Mutual PLC's (LON:OML) Quilter, and Phoenix Group PLC (LON:PHNX) owned Abbey Life.
The FCA said: "No decisions regarding these firms have been reached by the FCA at this stage.
“No inferences should be drawn from the closure of the Scottish Widows case concerning the continuing investigations. The FCA will update the market when decisions are made regarding the status of the remaining investigations."