The two-year contract extends a BPO arrangement the companies have had since 2000. Siemens will continue to manage areas of Barclays' retail banking back-office operations and also those of its credit card subsidiary, Barclaycard.
Siemens' responsibilities include account closures, transfer of funds and administrative processing for direct debit and standing order instructions.
"Since the relationship with Siemens began, Barclays has enjoyed a high standard of service with the company. This relationship has allowed us to focus on our core business," said John Stewart, Barclays service manager, branch and customer account services.
Phil Heggie, Siemens global director of BPO financial services, said, "We have delivered operational efficiencies to Barclays, which has allowed it to release more people into front-office, customer-facing roles."
Siemens signed a £2bn outsourcing deal with the BBC last year.