The postal group said it was the biggest major business mail contract to return to the organisation since a new regulatory framework was brought in last year.
The contract, which it lost to TNT in 2007, involves the collection, sorting and delivery of statements on behalf of British Gas.
Royal Mail has also been awarded a contract to deliver smart meters to British Gas engineers, and an extension to a deal for the overnight delivery of parts to the same company.
Mike Newnham, Royal Mail's chief customer officer, said: "Royal Mail is delighted to have won a competitive tendering process to collect, sort and deliver British Gas's statement mail.
"In a competitive postal market, Royal Mail's reputation, competitiveness and strength of account management were instrumental in securing this contract win."
Rob Morton of British Gas said: "We've set Royal Mail challenging objectives to ensure our customers get the best service and value, and look forward to working with them in the coming years."