The FCA said State Street UK's transitions management business had developed and executed a deliberate strategy to charge clients substantial mark-ups on certain transitions, in addition to the agreed management fee or commission.
These mark-ups had not been agreed by the clients and were concealed from them.
Between June 2010 and September 2011 the FCA found that State Street UK's TM business deliberately overcharged six clients a total of $20,169,603.
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State Street UK's clients include large investment management firms and pension funds holding the funds and savings of retail investors.
TM is a service provided to clients to support structural changes to asset portfolios with the intention of managing risk and increasing returns.
TM services may be required when a client needs a large portfolio of securities to be restructured, or when a client decides to remove or replace asset managers.
Tracey McDermott, director of enforcement and financial crime, said the firm acted with "complete disregard for the interests of its customers".
She said: "State Street UK allowed a culture to develop in the UK TM business which prioritised revenue generation over the interests of its customers.
"State Street UK's significant failings in culture and controls allowed deliberate overcharging to take place and to continue undetected. Their conduct has fallen far short of our expectations."
The FCA viewed State Street UK's failings to be at the most serious end of the spectrum.
Had it not agreed to settle at an early stage of the investigation and qualified for a 30% discount its fine would have been £32,692,800.