The seven-year deal, which begins next month, is to educate employers about the automatic enrolment of staff into workplace pension schemes.
The new regulations are being rolled out between October next year and September 2016. The regulator has to make contact with more than one million employers to make sure they follow the rules.
As well as issuing fixed penalty notices, Capita will serve compliance notices, run a customer contact centre and mange the process in which employers register with the regulator. More complex levels of enforcement will still be controlled by the watchdog, it added.
Pensions minister Steve Webb said the changes will help millions of people to save for retirement.