The reduction of the branch network in Guernsey will come into effect from 7 January 2021, says NatWest International.
Two branches apiece are also closing in Jersey and the Isle of Man, while the banking group is looking to reduce the headcount by 10% across the business - excluding Luxembourg.
The changes come as NatWest International says more people use digital and online banking.
'In recent years we’ve seen significant changes in customer behaviour with more customers choosing digital for their everyday banking needs,' said Andrew McLaughlin, chief executive officer of parent company RBS International.
'We must rebalance investment between our branch network and our new digital network in recognition of the way customers wish to bank with us.
'Customers will still have the option to visit our flagship branch in St Peter Port or speak with colleagues in our local banking telephony centre.'
He added: 'We also have a dedicated telephony service for vulnerable customers in the community. No branch is busier than our mobile banking service and today’s announcement recognises our need to move with the times'
The closures also bring with them the opening of a voluntary redundancy register across the bank. NatWest International said it was working with colleagues and trade unions to support colleagues affected by these changes.
At the height of the Covid-19 pandemic, NatWest closed its High Street branch to protect customers, which was near the end of March,
Digital options were encouraged while the Cobo and St Sampson’s branches stayed open.
The Cobo branch was then temporarily closed earlier this summer, with the bank saying it was in discussions with the landlord of the premises with the lease due to come to an end in January.
At the time, NatWest said it was reviewing on how it used its branches and would update customers as soon as it had more information.
Rob Girard, island director of parent company RBS International, also spoke about the Cobo branch which was closed temporarily to allow the Town branch to reopen and to offer increased services at the Bridge branch, both of which have substantially larger banking halls. This was to allow for more ‘socially acceptable distancing’ and broader service provision.
But in a sign of customer concern, a sign was taped on the door of the Cobo branch which read: ‘Please reopen our Cobo branch.
‘Your customers need you; we will continue to hope to see you soon. We miss you. Please reopen soon.’