HSBC has told customers the St Sampson's branch will close permanently on 24 September.
The branch on the North Side of the Bridge will close after HSBC carries out a five month refurbishment of its bank in St Martin's.
That refit will be begin in late February.
HSBC say there will not be any job losses as a result of the St Sampson's closure.
Alvaro Teixeira, who is head of wealth and personal banking in the Channel Islands, blames the increasing popularity of mobile and online banking for the decision to close one of its three remaining Guernsey outlets:
"While we have seen a decline in the number of customers using our branches, we've developed a model that provides islanders with both the opportunity for face-to-face servicing, as well as online banking products and services."
The bank reassures customers that they will be able to access in-person banking in St Peter Port, and again when St Martin's reopens in August.
HSBC says the refurbishment is an investment in its Guernsey operation and plans include a modernised customer area, private meeting rooms and a high degree of automation of banking services.
The bank says it realises that change can be concerning for customers, and its staff are available to address any worries.
The Alderney and Sark branches are unaffected.
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