The Orpington-based arm of the Swedish group won the seven-year contract, which includes the production of 2m-plus households bills.
Stralfors will also handle additional print and mailing relating to the Southern Water customer base.
John Carter, group managing director of Stralfors UK, said: "We believe the investment to obtain the accreditation of ISO27001 has proven our commitment for working with businesses that manage highly sensitive information."
In March, Southern Water began a comprehensive pre-qualification process with a number of leading transaction mail providers before awarding the tender.
In addition to the compulsory requirements, Southern Water demanded ISO9001, 14001 and 27001 accreditation and account management procedures.
Kim Salmon, director of revenue at Southern Water, said: "Our customers are of great value to our business and it was essential that we work with a partner that can not only supply the volume of transaction communications we require but also ensure the security of customer data."
Stralfors wins 1.7m Southern Water print contract
Stralfors has been awarded a 1.7m contract with Southern Water to supply print and mail services for its customer billing and other associated communications strategy, following a competitive tender process.
