Clydesdale Bank this week announced that the town branch will shut on April 25.
And their Stranraer branch will close down a day later.
The company explained it is part of a bid to create an integrated digital, mobile, telephone and branch service for customers.
And while confirming potential job losses, they also stressed an intention to find roles for staff within other branches.
South Scotland MSP Colin Smyth has described the move as “a bombshell” for communities.
He said: “It’s a major blow to the bank’s customers in Stranraer and Castle Douglas and will be devastating for those who will lose their job.
“There are now just two Clydesdale Banks left in the region miles apart so it is pie in the sky for the company to claim they will be able to find staff jobs elsewhere.”
He has launched a petition against the closures and added: “The message they send out to customers in rural areas is that they’re just not interested in them.”
Meanwhile, Dumfries and Galloway MP Richard Arkless said: “I am deeply disappointed by the announcement of the Clydesdale Bank.
“The closure of a bank has a huge impact on our small towns as a whole, negatively impacting footfall and damaging all of the surrounding businesses.”
He is hoping to arrange a meeting with bank officials soon and appealed for affected customers to get in touch with his office.
Explaining the decision, Clydesdale Bank spokesman Gavin Opperman said: “While the decision to close any branch is never an easy one, it is important that we, in line with other banks operating in the UK market, continue to respond to changes in the way customers want to bank with us.
“The changes announced today continue our journey towards a model that combines an enhanced digital platform with a right sized branch network; allowing customers to interact with us through a wide range of channels.”