The Spanish bank, which rebranded the high street branches of Abbey National, Alliance & Leicester and Bradford & Bingley last year, combined its retail and corporate banking PR and CSR work into one single account.
Citigate won the combined brief after a three-way competitive pitch - Citigate had previously worked with Santander to handle its commercial and retail banking, while Grayling formerly managed the bank's CSR account.
The brief is thought to be worth up to £200,000 a year, but it is understood that the bank was keen to keep a tight grip on costs associated with its PR programme.
The pitch process was hand-led by third-party procurement firm Aquanina, which is owned by Santander. The pitch was driven by procurement policy and was understood to be a mandatory review. One source not involved in the final pitch process said that six agencies were required to go through an online 'bidding' process to remain competitive on costs.
Another observer added that Santander wanted the successful agency to perform a significant amount of work for the fees it was prepared to pay.
A key aspect of the brief is to boost the brand's involvement in corporate Britain. Santander has made notable strides in retail banking, but wants to diversify from personal finance towards commercial and SME customers.
The review follows significant changes at Santader. In December 2010, Ana Botin replaced Lloyds-bound Antonio Horta-Osorio as UK CEO.
Several months later, comms director Matthew Young followed him to Lloyds, to be replaced by Jennifer Scardino.
It emerged before Christmas that Horta-Osorio had received casual advice from Brunswick around his return to Lloyds after sick leave.